Crossword Genius

Fish turned up lacking a head (3)

Ross

I believe the answer is:

nut

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'head' is the definition.
(informal term for somebody's head)

I don't understand how the rest of the clue works.

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(Other definitions for nut that I've seen before include "Bonce" , "1A [CRACKPOT]?" , "Hard fruit or informal lunatic" , "Bolt counterpart" , "Fruit with hard shell" .)

I've seen this clue in The Times and The Sunday Times.
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