Crossword Genius

When a Frenchman joins the gang, it's a bad thing (8)

Ross

I believe the answer is:

gangrene

I'm a little stuck... Click here to teach me more about this clue!

'gang it's a bad thing' is the definition.
I can't tell whether this definition defines the answer.

'when a frenchman joins' is the wordplay.
'when' becomes 'gang' (I can't explain this - if you can you should believe this answer much more).
'a frenchman' becomes 'rene' (common French name).
'joins' says to put letters next to each other.
'gang'+'rene'='GANGRENE'

'the' is the link.

Can you help me to learn more?

(Other definitions for gangrene that I've seen before include "It's mortifying" , "Localised tissue decay from infection or lack of blood supply" , "Mortify" , "Death of part of the body" , "Local death of body tissue" .)

I've seen this clue in the Evening Standard.
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