Psalm 110:7 "He shall drink of the brook in the way: therefore shall he lift up the head."
It seems almost out of place considering the Psalm seems to be talking about God. So what does it mean? Metaphor? for what?
The psalm appears to me to be talking about the king of Israel.1 While God is mentioned throughout Psalm 110, it is as the one who empowers Israel's king, who is the central figure of the song:
There is a loose parallelism in play, with the first three points being similar in structure to the next three points; water comes into play in the third and final points of the psalm (dew, and a drink from the brook).
Because verse 7 follows right after the atmosphere of war in verses 5-6, we should follow that idea. The act of the king raising up his head after taking a drink is perhaps a picture that the king has already achieved the victory over the enemies mentioned in the previous verses. This would be reinforced if there is indeed a parallel between the psalms two halves; verses 1-2 show the king being exalted by God, with verse 3 being his 'victory', and then verses 5-6 again show the king being exalted by God, with verse 7 being his 'victory'.
This is suggested in Zorn's commentary on the psalm:
Other commentators, while still seeing the psalm as a war-victory song for the king of Israel, see in verse 7 an allusion to the anointment of Solomon as king by the river Gihon (1 Kings 1.38-40).3 The psalm, then, might be understood as intended to be sung at the 'enthronement ceremonial rite'4 of a new king in Israel, with the king emerging from his anointment with his head held high.
In anticipation of the newly-anointed king's rule, the psalm invests a variety of Israelite ideas into the king (i.e. that he would be a ruler like Melchizedek, a ruler like David, a ruler like Solomon).
1 It has ideas similar to Psalm 2. In that psalm, God installs his 'messiah' the king of Israel in Zion (2.6; cf. 110.1-2), who wields an iron rod (2.9; cf. 110.2) and strikes down nations that oppose him (2.1-3,10-12; cf. 110.5-6).
2 Walter D. Zorn, Psalms Volume 2 (2004), p.326.
3 For example: John Goldingay, Psalms: Psalms 90-150 (2008), p.298. David M. Carr, Colleen M. Conway, An Introduction to the Bible: Sacred Texts and Imperial Contexts (2010), p.66.
4 Walter D. Zorn, Psalms Volume 2 (2004), p.325-326, though he actually argues against this view.
There are many answers to be found from numerous expositors. What has been proposed which best strikes the mood (apart from the view proposed by many others as a refreshed victor) is the picture of a warrior 'in pursuit' who has no time to worry about comforts, or even knell down to refresh himself. We might recall the ready warriors who did not kneel to drink but quickly lapped up water on the ready.
This line of thought goes way back. For example Calvin took this position.
I actually fond this explanation more persuasive because if it symbolizes victory I feel it is an anticlimactic statement compared to the previous verse. On the other hand if after the climatic declaration oh his total victory over all nations, this is an accent to gives feeling into the manner and spirit of how he will achieve the victory, it becomes quite riveting and complimentary to the previous climax. A additional climax if you will.