Side with running mate (4)
I believe the answer is:
team
'side' is the definition.
(thesaurus)
'running mate' is the wordplay.
'running' indicates anagramming the letters.
'mate' is an anagram of 'TEAM'.
'with' acts as a link.
(Other definitions for team that I've seen before include "One side in a game" , "Side; harness (horses)" , "Group pulling together" , "Side in sporting event" , "Competitive side" .)