Sample of pure substance knocked back by consumer (4)
I believe the answer is:
user
'consumer' is the definition.
(user is a kind of consumer)
'sample of pure substance knocked back' is the wordplay.
'sample of' indicates the answer is hidden within the clue (I've seen this in other clues).
'knocked back' says the letters should be written backwards.
'resu' is hidden in the letters of 'pure substance'.
'resu' back-to-front is 'USER'.
'by' acts as a link.
(Other definitions for user that I've seen before include "Operator; drug addict" , "Ruse for employer" , "Drug addict (informal)" , "Manipulative woman" , "Manipulator" .)