Study held by agents backing agricultural machine (8)
I believe the answer is:
spreader
'agricultural machine' is the definition.
(machine for spreading things in farming)
'study held by agents backing' is the wordplay.
'study' becomes 'read' (synonyms).
'held by' is an insertion indicator.
'agents' becomes 'reps' (representatives).
'backing' says the letters should be written in reverse.
'reps' backwards is 'sper'.
'read' going into 'sper' is 'SPREADER'.
(Other definitions for spreader that I've seen before include "that covers a wide area" , "Device for applying, eg, adhesive" .)