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' says the letters should be written in reverse.
'vips' in reverse letter order is 'SPIV'.
(Other definitions for spiv that I've seen before include "Sharp operator" , "Wide boy" , "Flash fellow" , "Flashy black-marketeer" , "Black market operator" .)