Drink Kitty mostly knocked back (3)
I believe the answer is:
sup
'drink' is the definition.
(to sup is to drink)
'kitty mostly knocked back' is the wordplay.
'kitty' becomes 'puss' (synonyms).
'mostly' means to remove the last letter (most but not all of the word).
'knocked back' shows that the letters should be reversed in order.
'puss' with its last letter taken away is 'pus'.
'pus' reversed gives 'SUP'.
(Other definitions for sup that I've seen before include "Drink -- eat (with the devil?)" , "Drink; take a late meal" , "Drink; eat evening meal" , "Take food" , "Eat a late meal; sip" .)