Crossword Genius

An urchin has it with a muffin (3)

Ross

I believe the answer is:

rag

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'an urchin has it' is the definition.
I can't judge whether this definition defines the answer.

'with a muffin' is the wordplay.
'with' means one lot of letters goes inside another.
'muffin' becomes 'rg' (I can't explain this - if you can you should give a lot more credence to this answer).
'a' put inside 'rg' is 'RAG'.

Can you help me to learn more?

(Other definitions for rag that I've seen before include "Group of colts" , "Piece of music" , "Student fundraiser" , "charity event" , "A tattered cloth or university frolic" .)

I've seen this clue in the Evening Standard.
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