Morticia, to Fester (5)
I believe the answer is:
niece
'morticia to fester' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for niece that I've seen before include "girl further down the line" , "[PRIEST'S] daughter?" , "Female family member" , "Brother's daughter" , "One of the family" .)