Be in league to get the side promoted (4,2)
I believe the answer is:
team up
'be in league' is the definition.
(thesaurus)
'get the side promoted' is the wordplay.
'get' means one lot of letters go next to another.
'the side' becomes 'team' (I've seen this before**).
'promoted' becomes 'up' (I've seen this in other clues).
'team' put next to 'up' is 'TEAM UP'.
'to' is the link.
(Other definitions for team up that I've seen before include "Couple" , "Join" , "Unite for a common goal" , "Brighton and Hove Albion following promotion?" .)