Crossword Genius

Billy's stupid rhyme (5)

Ross

I believe the answer is:

silly

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

This might be a double definition.

'billy's stupid' is the first definition.
'silly' can be an answer for 'stupid' (I've seen this before). I am not sure about the 'billy's' bit.

'rhyme' is the second definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

Can you help me to learn more?

(Other definitions for silly that I've seen before include "Not serious" , "near the batsman" , "Trivial, frivolous" , "Potty or barmy" , "Ridiculous, frivolous" .)

I've seen this clue in the Evening Standard.
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