Join forces every year (4)
I believe the answer is:
ally
'join forces' is the definition.
(to ally with someone is to join forces with them)
'every year' is the wordplay.
'every' becomes 'all' (similar in meaning).
'year' becomes 'y'.
'all'+'y'='ALLY'
(Other definitions for ally that I've seen before include "Cooperating country" , "Friend and colleague" , "One on one's side" , "Fighting partner" , "State formally cooperating with another" .)