How you choose a fabric ... (2,4)

I believe the answer is:

at will

'how you choose' is the definition.

'a fabric' is the wordplay.
'fabric' becomes 'twill' (twill is a kind of fabric).
'a'+'twill'='AT WILL'

(Other definitions for at will that I've seen before include "Whenever you please, as you like" , "Fire .. .... - whenever you're ready" , "As one chooses" , "Whenever one wishes" , "take it as I please" .)

I've seen this clue in The Times.

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