Pass the ball to an opponent? (5)
I believe the answer is:
serve
'pass' is the definition.
(serving is a kind of passing)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for serve that I've seen before include "Minister to" , "get game started" , "Start (tennis game)" , "Start a tennis match" , "Work for, be of use to" .)