Artist's agent rejecting one duty in five (8)
I believe the answer is:
fixative
'artist's agent' is the definition.
(I've seen this before)
'rejecting one duty in five' is the wordplay.
'rejecting' says the letters should be written in reverse.
'one' becomes 'i' (Roman numeral).
'duty' becomes 'tax' (I've seen this before).
'in' is an insertion indicator.
'i'+'tax'='itax'
'itax' written backwards gives 'xati'.
'xati' placed into 'five' is 'FIXATIVE'.
(Other definitions for fixative that I've seen before include "Artist's agent" , "Stabilising agent" .)