Officer material getting a green beret, finally (8)
I believe the answer is:
sergeant
'officer' is the definition.
(sergeant is a kind of officer)
'material getting a green beret finally' is the wordplay.
'material' becomes 'serge' (type of fabric).
'getting' is a charade indicator (letters next to each other).
'finally' suggests the final letters.
The last letters of 'green beret' are 'nt'.
'serge'+'a'+'nt'='SERGEANT'
(Other definitions for sergeant that I've seen before include "NCO; Death a fell one, Hamlet" , "officer of the law" , "Military rank" , "Reagents (anag)" , "Copper" .)