Celebrities backing black marketeer (4)
I believe the answer is:
spiv
'black marketeer' is the definition.
(I've seen this before)
'celebrities backing' is the wordplay.
'celebrities' becomes 'vips' (I've seen this before).
'backing' shows that the letters should be reversed in order.
'vips' back-to-front is 'SPIV'.
(Other definitions for spiv that I've seen before include "Sharp operator" , "Wide boy" , "Flash fellow" , "Flashy black-marketeer" , "Black market operator" .)