Barrel-maker had to join forces (9)
I believe the answer is:
cooperate
'join forces' is the definition.
(thesaurus)
'barrel-maker had' is the wordplay.
'barrel maker' becomes 'cooper' ('cooper' can be a synonym of 'barrel maker').
'had' becomes 'ate' (e.g. I had an ice cream).
'cooper'+'ate'='COOPERATE'
'to' is the link.
(Other definitions for cooperate that I've seen before include "Work to the same end" , "work hand in glove" , "band together" , "fit in with others" , "Work with another, be helpful" .)