To mum it's nonsense (4)

I believe the answer is:

tosh

'it's nonsense' is the definition.
(tosh is a kind of nonsense)

'to mum' is the wordplay.
'mum' becomes 'sh' (both are used to tell someone to be quiet).
'to'+'sh'='TOSH'

(Other definitions for tosh that I've seen before include "moonshine" , "Baloney, twaddle" , "[BLAH]" , "Nonsense (colloq.)" , "garbage'" .)

I've seen this clue in The Telegraph.

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