Stuff to knock back? It’ll make you tiddly (4)
I believe the answer is:
marc
'it'll make you tiddly' is the definition.
(marc is an alcoholic spirit)
'stuff to knock back?' is the wordplay.
'stuff' becomes 'cram' (cramming is a kind of stuffing).
'to knock back?' shows that the letters should be reversed in order (I've seen 'knock back' mean this).
'cram' backwards is 'MARC'.
(Other definitions for marc that I've seen before include "Cram (anag.)" , "Grape refuse" , "Brandy made from fruit-refuse" , "Type of eau-de-vie made from residue of grapes" , "A brandy; --- Chagall, artist" .)