A rifle emptied, held by criminal element (6)
I believe the answer is:
carbon
'element' is the definition.
(carbon is a kind of element)
'a rifle emptied held by criminal' is the wordplay.
'rifle' becomes 'rob' (rifle can mean to steal or rob).
'emptied' suggests removing the centre.
'held by' indicates putting letters inside.
'criminal' becomes 'con' (abbreviation for 'convict').
'rob' with its middle removed is 'rb'.
'a'+'rb'='arb'
'arb' going within 'con' is 'CARBON'.
(Other definitions for carbon that I've seen before include "Element in soot, charcoal, etc" , "Eg, diamond or graphite" , "Substance" , "Element of glass" , "Element of which diamonds are formed" .)