Criminal's food sent back; mostly fat (7)
I believe the answer is:
burglar
'criminal's' is the definition.
(burglar is a kind of criminal)
'food sent back mostly fat' is the wordplay.
'food' becomes 'grub' (grub can informally mean food).
'sent back' shows that the letters should be reversed in order.
'mostly' means to remove the last letter (most but not all of the word).
'fat' becomes 'lard' (lard is a kind of fat).
'grub' reversed gives 'burg'.
'lard' with its final letter removed is 'lar'.
'burg'+'lar'='BURGLAR'
(Other definitions for burglar that I've seen before include "Thief (entering building)" , "House robber" , "Breaker and enterer?" , "One enters a building in order to steal" , "A criminal on one's premises" .)