Diminish Batman, vocally (4)

I believe the answer is:

wane

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'diminish' is the definition.
(I know that wane is a more specific form of the action diminish)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for wane that I've seen before include "Decline (in influence)" , "Grow smaller like the moon" , "Decrease (like the moon)" , "Opposite of wax" , "Become smaller" .)

I've seen this clue in the Sydney Morning Herald.

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