Scruff using pot rejected Ecstasy (4)
I believe the answer is:
nape
'scruff' is the definition.
(synonyms)
'pot rejected ecstasy' is the wordplay.
'pot' becomes 'pan' (I've seen this in other clues).
'rejected' says the letters should be written in reverse.
'ecstasy' becomes 'E' (informal term for the drug).
'pan' in reverse letter order is 'nap'.
'nap'+'e'='NAPE'
'using' is the link.
(Other definitions for nape that I've seen before include "Here's a hairline" , "Scruff of the neck" , "Scrag" , "It's a pane in the neck" , "sofa" .)