Military wear is the antithesis of a baby's bonnet! (6,3)

I believe the answer is:

forage cap

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But I can't explain it at all! Could you help me understand?

(Other definitions for forage cap that I've seen before include "item of military kit" , "Fore-and-aft military headgear" , "hat" , "Soldier's covering" , "Infantryman's headgear" .)

I've seen this clue in the Evening Standard.

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