Playing lines United used to replace energy once (6)
I believe the answer is:
versus
'playing' is the definition.
(as in a sports team playing against another)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for versus that I've seen before include "Set against" , "Against in games" , "Preposition meaning against, as in contest" , "playing" , "v. (sport)" .)