Collar gunmen's oil supplier (5)
I believe the answer is:
copra
'oil supplier' is the definition.
(coconut oil comes from copras)
'collar gunmen's' is the wordplay.
'collar' becomes 'cop' (both can mean to apprehend or catch).
'gunmen' becomes 'RA' (Royal Artillery).
'cop'+'ra'='COPRA'
(Other definitions for copra that I've seen before include "Nutty product" , "Oil producer" , "What gives oil" , "source of oils" , "Dried coconut kernels" .)