One of the gang getting the rest upset? (7)

I believe the answer is:

mobster

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'one of' is the definition.
The definition suggests a singular noun which matches the answer.

'gang getting the rest upset?' is the wordplay.
'gang' becomes 'mob' (synonyms).
'getting' means one lot of letters go next to another.
'upset?' indicates an anagram (I've seen 'upset' mean this).
'rest' anagrammed gives 'ster'.
'mob'+'ster'='MOBSTER'

'the' is the link.

Can you help me to learn more?

(Other definitions for mobster that I've seen before include "Hood" , "Mafioso" , "Gangster" , "family man" , "Member of criminal organisation" .)

I've seen this clue in the Evening Standard.

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