It's a possibility for a drinker (4)
I believe the answer is:
asti
'a drinker' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'it's a possibility' is the wordplay.
'it's a' anagrams to 'ASTI'.
However, I am not sure how this is indicated.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for asti that I've seen before include "18" , "Piedmont wine city, SE of Turin" , "Italian white wine" , "Source of fizz" , "It's a (anag) - wine" .)