Mate running to the side (4)
I believe the answer is:
team
'the side' is the definition.
(a side is a sporting team)
'mate running' is the wordplay.
'running' is an anagram indicator (the letters run around to different places).
'mate' is an anagram of 'TEAM'.
'to' is the link.
(Other definitions for team that I've seen before include "(Football) side" , "Players united" , "(Two or more) horses in harness" , "A set of players or workers" , "eleven?" .)