Fool of a girl who lost her head (3)

I believe the answer is:

ass

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'fool' is the definition.
(both can mean a stupid person)

I cannot understand how the remainder of the clue works.

(Other definitions for ass that I've seen before include "Quadruped; fool" , "transporter" , "Hoofed animal, smaller than a horse" , "Braying animal" , "Blockhead" .)

I've seen this clue in the Evening Standard.

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