Ace maybe to be employed in Forces (5)
I believe the answer is:
serve
This clue is a double definition.
'ace maybe' is the first definition.
(tennis term)
'to be employed in forces' is the second definition.
(Other definitions for serve that I've seen before include "Be adequate; put ball in play" , "Milton 'They also . . . . . who only stand and wait'" , "Commence play in tennis" , "Wait - start the play" , "Assist" .)