Girl's not much wearing fancy hat (7)
I believe the answer is:
Here is my best explanation:
'girl's' is the definition.
'not much wearing fancy hat' is the subsidiary indication.
'not much' becomes 'bit'.
'wearing' means one lot of letters goes inside another.
'fancy hat' becomes 'taha' (I can't justify this - if you can you should give a lot more credence to this answer).
'bit' inserted inside 'taha' is 'tabitha'.
(Other definitions for tabitha that I've seen before include "female", "girl", "Girl" and "Girl's name".)