Adherent is one to drink beer (9)
I believe the answer is:
supporter
'adherent' is the definition.
(I've seen this before)
'one to drink beer' is the wordplay.
'one to drink' becomes 'sup' (to sup is to drink).
'beer' becomes 'porter' (type of beer).
'sup'+'porter'='SUPPORTER'
'is' acts as a link.
(Other definitions for supporter that I've seen before include "Fan; heraldic figure" , "Stay" , "One helping" , "Champion" , "Second" .)