Fish turned up lacking a head (3)
I believe the answer is:
nut
'head' is the definition.
(informal term for somebody's head)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for nut that I've seen before include "Bonce" , "1A [CRACKPOT]?" , "Hard fruit or informal lunatic" , "Bolt counterpart" , "Fruit with hard shell" .)