Rissole obviously contains fat (4)
I believe the answer is:
oleo
'fat' is the definition.
(thesaurus)
'rissole obviously contains' is the wordplay.
'contains' indicates the answer is hidden within the clue.
'OLEO' can be found hidden inside 'rissole obviously'.
(Other definitions for oleo that I've seen before include "Prefix indicating oily" , "Oil used in making margarine" , "Fatty substance" , "Marge" .)