Backer to drink beer (9)
I believe the answer is:
supporter
'backer' is the definition.
(thesaurus)
'drink beer' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'beer' becomes 'porter' (type of beer).
'sup'+'porter'='SUPPORTER'
'to' is the link.
(Other definitions for supporter that I've seen before include "One cheers" , "Second" , "Fan; heraldic figure" , "angel" , "Strut" .)