Dye covering half of hair in helmet (3,3)
I believe the answer is:
Here is my best explanation:
'helmet' is the definition.
(tin hat is a kind of helmet)
'dye covering half of hair' is the subsidiary indication.
'dye' becomes 'tint' (tinting is a kind of dyeing).
'covering' indicates putting letters inside.
'half of' means to take half the letters.
'hair' halved 'ha'.
'tint' going around 'ha' is 'tinhat'.
'in' is the link.
(Other definitions for tin hat that I've seen before include "Soldier's helmet", "Steel helmet (informal)", "Protective helmet once worn by constuction and mine workers", "Finish it off to put the protective headgear on it" and "Headgear, worn for protection".)