Respond concerning part of play (5)
I believe the answer is:
react
'respond' is the definition.
(I've seen this before)
'concerning part of play' is the wordplay.
'concerning' becomes 're'.
'part of play' becomes 'act' ('act' is part of 'play').
're'+'act'='REACT'
(Other definitions for react that I've seen before include "Retaliate" , "Respond to something" , "provide an answer" , "Hit back" , "Acknowledge" .)