Singular clothing can never be all the rage! (3)

I believe the answer is:

rag

I'm a little stuck... Click here to teach me more about this clue!

'singular clothing' is the definition.
'rag' can be an answer for 'clothing' (thesaurus). I am unsure of the 'singular' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for rag that I've seen before include "University festival or torn cloth" , "Waste scrap of cloth" , "charity event" , "organ?" , "Waste piece of cloth" .)

I've seen this clue in the Evening Standard.

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