But it could be one to play (4)
I believe the answer is:
work
'one to play' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for work that I've seen before include "Tasks to be undertaken" , "Wildly happy" , ". . . get on with serious business" , "Function; task" , "Toil" .)