Q: In John 6:29- When Jesus answered, this is the "work ((ἔργον) of God"- Is it a wordplay"
1. This is the work of God
The "work of God" taken with God in the possessive genitive means "work wrought by God, the work God has done." However, contextual clues indicate that the proper interpretation would be, this is the work that God requires. In verse 28, they asked Jesus about what they needed to do, not what God needed to do for them. And in v. 27, Jesus told them what they need to do, which set the context for the subsequent verses. An analogy would be, to workers who want employment, the vineyard owner says, "my work" (what I require of you) for you is to pick grapes."
It is noteworthy how John Calvin comments: "They had spoken of works Christ reminds them of one work, that is, faith; ... but that faith alone is sufficient because this alone does God require from us, that we believe." Here, he takes the work of God to mean the work that God requires from us.
Constable's note also says, "The only work that God requires of people for salvation is faith in His Son (cf. John 3:11-17). The work that Jesus specified was not something physical at all. It was what God requires; namely, trust in Jesus (cf. Romans 3:28)."
NET Bible rightly translates v.29 as, "This is the deed (ἔργον) God requires—to believe (ἵνα πιστεύητε - that you should believe) in the one whom He sent. (Note: annotation mine)
2. And how can "πιστεύητε" in Him be "work" (ἔργον)?
The πιστεύητε (present subjunctive active) means continue believing and trusting in Jesus. It implies a dynamic way of life, moving from faith to faith, living by faith (Rom. 1:17), "working out your salvation" (Phil. 2:12), and "ὁ νικῶν" (overcoming, Rev. 21:7)--the typical depiction of a living faith (James 2:14-26). The πιστεύω also has the concept of faithfulness and loyalty to whom we believe.
It involves our will to have lifelong commitment and dedication in obedience to Holy Spirit and Words. In that regard, πιστεύητε is work (ἔργον) not as a means to earn salvation, but the manifestation of the life of the living faith in the Lord and Savior. And it is "active co-working" with God. God, at all costs, respects our will and expects our willing participation in doing our part. God has all the provisions ready for His children, the Word, the angel (Heb. 1:14), and the Holy Spirit (Παράκλητος -Helper). It takes a humble heart submitting to the Lord, yielding to, yearning for His mercy and grace without ceasing, and confessing for our lapses in doing so. Sovereign God can but will not infringe the part of our responsibility. When we cry out in our weakness, He will extend His hand to rescue us—i.e., drowning Peter (Matt. 14:31).
For Mitzvah practicing Jews, it is a radical statement: setting free from the burdensome works of 613 do's and don'ts to one work of believing in Jesus, who came to fulfill the Law (Matt. 5:17-20), on the cross He finished it (John 19:30).
Believing in Him involves our will to have lifelong commitment and dedication in obedience, and that is the work God requires of us, and we take our cross and follow Him (Matt. 16:24).
And, it was not a wordplay!