Frenchman with brogue's half Welsh (7)
I believe the answer is:
renegue
'welsh' is the definition.
(I've seen this before)
'frenchman with brogue's half' is the wordplay.
'frenchman' becomes 'rene' (common French name).
'with' is a charade indicator (letters next to each other).
'half' means to take half the letters.
'brogue' cut in half is 'gue'.
'rene'+'gue'='RENEGUE'
(Other definitions for renegue that I've seen before include "Go back on one's word" , "Fail to fulfil an obligation" , "Go back on (a promise)" , "Deny" , "Fail to fulfil a promise" .)