Being on edge, he starts mumbling (3)

I believe the answer is:

hem

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'being on edge' is the definition.
'hem' can be an answer for 'edge' (thesaurus). I am not certain of the 'being on' bit.

'he starts mumbling' is the wordplay.
'starts' indicates taking the first letters.
The initial letter of 'mumbling' is 'm'.
'he'+'m'='HEM'

Can you help me to learn more?

(Other definitions for hem that I've seen before include "Stitched edge of garment" , "Sewn edge to fabric" , "Express hesitation" , "A stitched border of cloth" , "It draws attention to" .)

I've seen this clue in the Evening Standard.

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