Contents of trophy idiot filled with inert gas (5)
I believe the answer is:
ashes
'contents of trophy' is the definition.
(as in the Ashes cricket trophy)
'idiot filled with inert gas' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'filled with' means one lot of letters goes inside another.
'inert gas' becomes 'He' (chemical symbol for helium, an inert gas).
'ass' placed around 'he' is 'ASHES'.
(Other definitions for ashes that I've seen before include "Former Wimbledon champion's" , "Remains of anything burnt" , "''... to ..., dust to dust'' (burial service)" , "Embers" , "Residue from fire" .)