A ropy rhyme is silly! (5)

I believe the answer is:

dopey

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'is silly' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(Other definitions for dopey that I've seen before include "Half asleep, or not too bright" , "Doc's slow-witted pal" , "One of Seven Dwarfs - not too bright" , "Idiotic" .)

I've seen this clue in the Evening Standard.

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