Fat girl comes to end of diet (4)
I believe the answer is:
suet
'fat' is the definition.
(suet is a kind of fat)
'girl comes to end of diet' is the wordplay.
'girl' becomes 'sue' (girl's name).
'comes to' means one lot of letters go next to another.
'end of' suggests the final letters.
The last letter of 'diet' is 't'.
'sue'+'t'='SUET'
(Other definitions for suet that I've seen before include "Hard fat from beef and sheep" , "Edible animal fat" , "Thin pancake" , "Fatty tissue" , "Fat used in foods" .)