Hard drug has edge, but you won't react with it (5,5,3)
I believe the answer is:
stiff upper lip
'you won't react with it' is the definition.
(as in not showing emotion)
'hard drug has edge' is the wordplay.
'hard' becomes 'stiff' (similar in meaning).
'drug' becomes 'upper' (a type of stimulating drug).
'has' is a charade indicator (letters next to each other).
'edge' becomes 'lip' (lip is a kind of edge).
'stiff'+'upper'+'lip'='STIFF UPPER LIP'
'but' acts as a link.
(Other definitions for stiff upper lip that I've seen before include "Very British stoicism" , "Uncomplaining stoicism needed" , "Evidence of stoicism" , "British self-restraint" , "Quality of stoicism" .)