Part of cargo, natural gas (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'part of cargo natural' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ARGON' can be found hidden inside 'cargo natural'.
(Other definitions for argon that I've seen before include "Organ (anag.)" , "Gaseous element" , "Noble gas, Ar" , "Gas, inert constituent of air" , "Inert gas, present in small amounts in the air" .)