Burnt residue on bottom of grim food (4)
I believe the answer is:
mash
'food' is the definition.
(mash is a kind of food)
'burnt residue on bottom of grim' is the wordplay.
'burnt residue' becomes 'ash' (I've seen this before).
'on' means one lot of letters go next to another.
'bottom of' suggests the final letters.
The last letter of 'grim' is 'm'.
'ash' after 'm' is 'MASH'.
(Other definitions for mash that I've seen before include "Pureed potatoes" , "Crush into a soft, pulpy state" , "Potato dish" , "dish of veg" , "Creamed potatoes" .)