Rodents will eat last of rotten food (5)
I believe the answer is:
mince
'food' is the definition.
(mince is a kind of food)
'rodents will eat last of rotten' is the wordplay.
'rodents' becomes 'mice' (mouse is a kind of rodent).
'will eat' is an insertion indicator.
'last of' suggests the final letters.
The last letter of 'rotten' is 'n'.
'mice' enclosing 'n' is 'MINCE'.
(Other definitions for mince that I've seen before include "meat that's prepared" , "Simper" , "Walk daintily" , "Chop into tiny pieces" , "Finely chopped beef" .)