A bitter brew that might go to a priest's head (7)
I believe the answer is:
biretta
'a bitter brew that might go to a priest's head' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a bitter brew' is the wordplay.
'brew' indicates anagramming the letters.
'a'+'bitter'='abitter'
'abitter' is an anagram of 'BIRETTA'.
Can you help me to learn more?
(Other definitions for biretta that I've seen before include "Clerical headgear" , "Cleric takes this on himself" , "Stiff cap worn by Roman Catholic clergy" , "Battier (anag.)" , "Roman Catholic priest's cap" .)