A policeman, an artist, and a bit of a nut (5)
I believe the answer is:
copra
'a bit of a nut' is the definition.
(I've seen this before)
'a policeman an artist' is the wordplay.
'a policeman' becomes 'cop' (slang term).
'an artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'cop'+'ra'='COPRA'
'and' acts as a link.
(Other definitions for copra that I've seen before include "Dried coconut meat" , "Dried coconut-kernel" , "Dried coconut kernel" , "source of oils" , "Coconuts provide" .)