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Arm broke, then leg, ultimately in shock (4,3)

Ross

I believe the answer is:

sten gun

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'arm' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's a link between them I don't understand?

'broke then leg ultimately in shock' is the wordplay.
'ultimately' indicates one should take the final letters.
'in' indicates putting letters inside.
'shock' becomes 'stun' (both can mean to surprise).
The last letters of 'broke then leg' are 'eng'.
'eng' going into 'stun' is 'STEN GUN'.

Can you help me to learn more?

(Other definitions for sten gun that I've seen before include "One fires" , "Killer" , "weapon" .)

I've seen this clue in The Guardian.
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