Thoroughly gross murmur? (5)

I believe the answer is:

sough

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'murmur?' is the definition.
(thesaurus)

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

(Other definitions for sough that I've seen before include "Murmur (of wind)" , "Noise of wind" , "Make whispering sound like wind in trees" .)

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

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