Tell why one is playing well (6)
I believe the answer is:
inform
'tell' is the definition.
(I've seen this before)
'why one is playing well' is the wordplay.
'why one' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'is playing well' becomes 'form' (as in a sportsperson who is playing 'on form').
'in'+'form'='INFORM'
(Other definitions for inform that I've seen before include "Pass on knowledge" , "'Apprise, notify (6)'" , "Notify or tell" , "Tell - shop" , "Put wise" .)