Drink a saint unconscious! (5)
I believe the answer is:
stout
'drink' is the definition.
(stout is a kind of drink)
'a saint unconscious' is the wordplay.
'a saint' becomes 'st'.
'unconscious' becomes 'out' ('out' can be similar in meaning to 'unconscious').
'st'+'out'='STOUT'
(Other definitions for stout that I've seen before include "Overweight; porter" , "Resolute" , "Kind of beer" , "pint" , "Dark beer; of heavy build" .)