“Rajbhog,” he explained, “is full of fat.” (4)
I believe the answer is:
ghee
'fat' is the definition.
(thesaurus)
'rajbhog he explained is full of' is the wordplay.
'is full of' indicates the answer is hidden within the clue (inside).
'GHEE' can be found hidden inside 'rajbhog he explained'.
(Other definitions for ghee that I've seen before include "type of butter" , "Clarified butter used in Indian cooking" , "Use it to cook" , "Indian form of butter" , "Form of clarified butter" .)