Sing free melody without metre (5)

I believe the answer is:

yodel

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'sing' is the definition.
(I know that yodel is a kind of sing)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for yodel that I've seen before include "Alpine mountain call" , "Sing in high pitch like Swiss mountain dwellers" , "Sing in Tyrolean fashion" , "Sing (in the Alps)" , "make an off-peak call" .)

I've seen this clue in The Times.

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