It’s nonsense, mate (4)

I believe the answer is:

tosh

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This clue might be a double definition.

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

'mate' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

Can you help me to learn more?

(Other definitions for tosh that I've seen before include "Scottish chum" , "Baloney, twaddle" , "[BLAH]" , "Stuff and nonsense" , "13 [TRIPE]" .)

I've seen this clue in the Evening Standard.

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