Girl’s casual trousers (5)

I believe the answer is:

jeans

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'trousers' is the definition.
(I know this)

'girl's casual' is the wordplay.
'girl' becomes 'Jean'.
'casual' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'jean'+'s'='JEANS'

Can you help me to learn more?

(Other definitions for jeans that I've seen before include "Trousers made of strong cotton, usually dyed blue" , "boys in France?" , "Denims" , "Denim trousers" , "Casual trousers, sometimes stonewashed?" .)

I've seen this clue in The Times.

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