Idiot biting girl’s head off (3)
I believe the answer is:
ass
'idiot' is the definition.
(I've seen this before)
'girl's head off' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'head off' means to remove the first letter.
'lass' with its first letter removed is 'ASS'.
'biting' acts as a link.
This may not be right. It may belong to another bit of the clue.
(Other definitions for ass that I've seen before include "Quadruped; fool" , "Horse-type animal" , "Silly fool or beast of burden" , "transporter" , "Nincompoop" .)