Agreement during Whitsun is ongoing (6)
I believe the answer is:
unison
'agreement' is the definition.
(unison is a kind of agreement)
'during whitsun is ongoing' is the wordplay.
'during' indicates the answer is hidden within the clue.
'UNISON' can be found hidden inside 'whitsun is ongoing'.
(Other definitions for unison that I've seen before include "Sameness in pitch" , "Simultaneous action" , "All act as one in this" , "All singing the same note" , "all players directed to the same pitch" .)