Union head (3)

I believe the answer is:

nut

This clue is a double definition.

'union' is the first definition.
(National Union of Teachers)

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

(Other definitions for nut that I've seen before include "Screwing block" , "Head; hard fruit" , "Cashew or hazel" , "Hard fruit or informal lunatic" , "Cashew, for example" .)

I've seen this clue in the Evening Standard.

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