Military helmet (informal) (3,3)

I believe the answer is:

tin hat

(Other definitions for tin hat that I've seen before include "Steel helmet (informal)" , "Headgear, worn for protection" , "Soldier's helmet" , "Protective helmet once worn by constuction and mine workers" , "Finish it off to put the protective headgear on it" .)

I've seen this clue in The Guardian.

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