Cooperate to get the side promoted (4,2)
I believe the answer is:
team up
'cooperate' is the definition.
(thesaurus)
'get the side promoted' is the wordplay.
'get' says to put letters next to each other.
'the side' becomes 'team' (I've seen this before**).
'promoted' becomes 'up' (I've seen this in other clues).
'team' next to 'up' is 'TEAM UP'.
'to' acts as a link.
(Other definitions for team up that I've seen before include "Join" , "Couple" , "Unite for a common goal" , "Brighton and Hove Albion following promotion?" .)