Trademark protecting a piece of headgear (3)

I believe the answer is:

tam

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'piece of headgear' is the definition.
'tam' can be an answer for 'headgear' (tam is a kind of headgear). I'm not sure about the 'piece of' bit.

'trademark protecting a' is the wordplay.
'trademark' becomes 'tm' (abbreviation for trademark).
'protecting' indicates putting letters inside (inserted letters are held or protected).
'tm' going around 'a' is 'TAM'.

Can you help me to learn more?

(Other definitions for tam that I've seen before include "Headgear" , "Scots hat" , "Scotsman's cap" , "TV audience ratings" , "Index rating for TV watching" .)

I've seen this clue in The Independent.

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