Playing well, worker, on grass (9)
I believe the answer is:
informant
'grass' is the definition.
(I know that grass can be written as informant)
'playing well worker on' is the wordplay.
'playing well' becomes 'in form' (to be 'in form' is to be at one's best in a sport).
'worker' becomes 'ant' (worker is a type of ant).
'on' is a charade indicator (letters next to each other).
'inform'+'ant'='INFORMANT'
(Other definitions for informant that I've seen before include "Snitch" , "Grass" , "source of dope" , "Imparter of knowledge" , "Telltale" .)