Mate playing on the side (4)
I believe the answer is:
team
'the side' is the definition.
(a side is a sporting team)
'mate playing' is the wordplay.
'playing' indicates anagramming the letters.
'mate' with letters rearranged gives 'TEAM'.
'on' acts as a link.
(Other definitions for team that I've seen before include "Two or more working together" , "A set of players forming one side" , "Group working in combination" , "See 22" , "One side in a game" .)