Drug adulterated? That's a blow (8)
I believe the answer is:
uppercut
'a blow' is the definition.
(uppercut is a kind of blow)
'drug adulterated?' is the wordplay.
'drug' becomes 'upper' (a type of stimulating drug).
'adulterated?' becomes 'cut' (as in a substance being 'cut' with another).
'upper'+'cut'='UPPERCUT'
'that's' is the link.
(Other definitions for uppercut that I've seen before include "Punch delivered vertically to chin" , "Boxing blow delivered to chin from below" , "Swinging blow from below aimed at an opponent's chin" , "Swinging blow directed at an opponent's chin" , "Type of punch" .)