Fat girl comes to end of diet (4)
I believe the answer is:
suet
'fat' is the definition.
(I know that suet is a type of edible 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' indicates one should take the final letters.
The final 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" .)