Cast drinking bitter, and staggering! (9)
I believe the answer is:
startling
'staggering' is the definition.
(I've seen this before)
'cast drinking bitter' is the wordplay.
'cast' becomes 'sling' (slinging is a kind of casting**).
'drinking' is an insertion indicator.
'bitter' becomes 'tart' (I've seen this before).
'sling' placed around 'tart' is 'STARTLING'.
'and' acts as a link.
(Other definitions for startling that I've seen before include "Astonishing or shocking" , "Causing shock" , "Unexpected" , "Very surprising, astonishing" , "Remarkable" .)