Mate plays on the side (4)
I believe the answer is:
team
'the side' is the definition.
(a side is a sporting team)
'mate plays' is the wordplay.
'plays' is an anagram indicator.
'mate' is an anagram of 'TEAM'.
'on' is the link.
(Other definitions for team that I've seen before include "Group working together" , "Players united" , "Side; harness (horses)" , "One side in a game" , "Squad; side" .)