Material your setter's kept at back of study (5)
I believe the answer is:
denim
'material' is the definition.
(denim is a kind of material)
'your setter's kept at back of study' is the wordplay.
'your setter's' becomes 'im' ('I'm', referring to the setter of the crossword).
'kept at back of' is a charade indicator (letters next to each other).
'study' becomes 'den' (I've seen this before).
'im' put after 'den' is 'DENIM'.
(Other definitions for denim that I've seen before include "Tough cotton fabric" , "Hard-wearing cloth" , "Hard-wearing cotton fabric" , "Twill cloth" , "dark blue material" .)