Able to be whistled? (7)

I believe the answer is:

tuneful

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'able' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

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(Other definitions for tuneful that I've seen before include "Melodious" , "Melodic" , "Euphonious" , "With a pleasing musical sound" , "Euphonic" .)

I've seen this clue in the Evening Standard.

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