When a Frenchman joins the gang, it's a bad thing (8)
I believe the answer is:
gangrene
'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" .)