Underwear I should slip in for restaurant (9)

I believe the answer is:

brasserie

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'for restaurant' is the definition.
(brasserie is a kind of restaurant)

'underwear i should slip in' is the wordplay.
'underwear' becomes 'bras'.
'should slip' becomes 'sere' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'i' placed within 'sere' is 'serie'.
'bras'+'serie'='BRASSERIE'

Can you help me to learn more?

(Other definitions for brasserie that I've seen before include "bar in Paris" , "French-style restaurant" , "Small restaurant serving alcohol as well as food" , "Where to eat" , "bar serving food" .)

I've seen this clue in The Guardian.

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