Profound rejection after initially stimulating drug (5)
I believe the answer is:
speed
'drug' is the definition.
(I know that speed is a type of drug)
'profound rejection after initially stimulating' is the wordplay.
'profound' becomes 'deep' (similar in meaning).
'rejection' says the letters should be written backwards (to reject is to send back).
'after' is a charade indicator (letters next to each other).
'initially' says to take the initial letters.
The initial letter of 'stimulating' is 's'.
'deep' written backwards gives 'peed'.
'peed' after 's' is 'SPEED'.
(Other definitions for speed that I've seen before include "Dispatch" , "Rate of progress" , "Travel quickly" , "Rate of movement" , "Swiftness" .)