Grass playing well before being cut (9)
I believe the answer is:
informant
'grass' is the definition.
(I've seen this before)
'playing well before being cut' is the wordplay.
'playing well' becomes 'in form' (to be 'in form' is to be at one's best in a sport).
'before' becomes 'ante' (I've seen this before).
'being cut' means to remove the last letter.
'ante' with its last letter taken off is 'ant'.
'inform'+'ant'='INFORMANT'
(Other definitions for informant that I've seen before include "Telltale" , "source of intelligence" , "Grass" , "One who sings" , "Imparter of knowledge" .)