Finally plays in long grass (6)
I believe the answer is:
snitch
'grass' is the definition.
(thesaurus)
'finally plays in long' is the wordplay.
'finally' says to take the final letters.
'long' becomes 'itch' (I've seen this before).
The last letters of 'plays in' are 'sn'.
'sn'+'itch'='SNITCH'
(Other definitions for snitch that I've seen before include "Telltale (sl.)" , "Inform on someone in slangy way" , "Inform on, tell tales" , "Grass" , "Whistleblower (informal)" .)