No longer in ancient headwear (3,3)

I believe the answer is:

old hat

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'no longer in' is the definition.
The definition suggests a singular noun which matches the answer.

'ancient headwear' is the wordplay.
'ancient' becomes 'old' (similar in meaning).
'headwear' becomes 'hat' (I've seen this before).
'old'+'hat'='OLD-HAT'

Can you help me to learn more?

(Other definitions for old hat that I've seen before include "Out of date thing" , "Shabby cover" , "Boringly familiar" , "not the gear for Ascot?" , "Dolly Varden perhaps?" .)

I've seen this clue in The Sun.

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