Not exactly a dirty song (5)

I believe the answer is:

ditty

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'song' is the definition.
(I know that ditty is a type of song)

I don't understand the rest of the clue.

(Other definitions for ditty that I've seen before include "Simple tune" , "Air" , "Light-hearted song" , "Little tune" , "Short simple song" .)

I've seen this clue in the Evening Standard.

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