Cryptically, golf is cheerful (6)
I believe the answer is:
upbeat
'cheerful' is the definition.
('upbeat' can be a synonym of 'cheerful')
'cryptically golf is' is the wordplay.
I cannot really understand how this works, but
'is' could be 'be' and 'be' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for upbeat that I've seen before include "Relentlessly optimistic" , "Promising" , "maestro communicates this to orchestra" , "Optimistic (colloq.)" , "in every bar" .)