The "A" of I.P.A. (3)
I believe the answer is:
ale
'the a of ipa' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ale that I've seen before include "Alcoholic drink" , "Sold in 23" , "Alcohol" , "Stout" , "Type of beer" .)