Piece of cake for girl in uniform (7)
I believe the answer is:
brownie
I believe this is a double definition.
'piece of cake' is the first definition.
(I've seen this before)
'girl in uniform' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for brownie that I've seen before include "Benevolent elf; cake" , "Girl cub - chocolate cake" , "Chocolate cake; elf" , "Junior member of girl's movement ..." , "Girl in uniform" .)