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Firm that originally rejected protective cover (4,3)

Ross

I believe the answer is:

hard hat

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'firm that originally rejected protective cover' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'firm that originally rejected protective cover' is the wordplay.
I cannot really understand how this works, but
'firm' could be 'hard' (both can mean durable or strong) and 'hard' is found in the answer.
'cover' could be 'hat' (cover for one's head) and 'hat' is present in the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for hard hat that I've seen before include "Protective headgear" , "Construction worker's helmet" , "Builder's safety wear" , "'nutcase'" , "Safety headgear" .)

I've seen this clue in The Independent.
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