Hat that can double as a gong! (3)

I believe the answer is:

tam

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'hat' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for tam that I've seen before include "Scots cap" , "Turn over the mat for a cap" , "'Scottish cloth cap, in short (3)'" , "TV audience ratings" , "'Woollen cap of Scottish origin, in short (3)'" .)

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