Crossword Genius

Rooney's caught Peter out (4)

Ross

I believe the answer is:

wane

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'peter out' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for wane that I've seen before include "Diminish" , "Decrease, decline" , "Decrease in size or power" , "Atrophy" , "Lose force, intensity" .)

I've seen this clue in The Independent.
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