Girl in the fam (3)
I believe the answer is:
sis
'girl in the fam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sis that I've seen before include "Little sibling" , "spies ..." , "Family member (informal)" , "Sibling (informal)" , "Secret Intelligence Service (abbr.)" .)