Join forces with side when aloft (4,2)
I believe the answer is:
team up
'join forces' is the definition.
(thesaurus)
'with side when aloft' is the wordplay.
'with' means one lot of letters go next to another.
'side' becomes 'team' (I've seen this before).
'when' becomes 'pu' (I am not sure about this - if you are sure you should believe this answer much more).
'aloft' is a reversal indicator.
'pu' back-to-front is 'up'.
'team'+'up' is 'TEAM UP'.
Can you help me to learn more?
(Other definitions for team up that I've seen before include "Couple" , "Unite for a common goal" , "Brighton and Hove Albion following promotion?" , "Join" .)