Base where drink held up by head (9)
I believe the answer is:
predicate
'base' is the definition.
(to be predicated on something is to be based on it)
'drink held up by head' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'at' and 'at' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'where' acts as a link.
(Other definitions for predicate that I've seen before include "Grammatically, the statement made about a subject" , "Found" , "Word in a sentence that make a statement about the subject" , "Claim" , "It says something about the subject in a sentence" .)