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 they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

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

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