Girl removing top is a fool (3)
I believe the answer is:
ass
'a fool' is the definition.
(both can mean a stupid person)
'girl removing top' is the wordplay.
'girl' becomes 'Cass'.
'removing top' means to remove the first letter (I've seen 'removes top' mean this).
'cass' with its first letter taken away is 'ASS'.
'is' is the link.
(Other definitions for ass that I've seen before include "Titania loved one" , "One in Bible spoke" , "Idiot; animal" , "Quadruped; fool" , "Blockhead" .)