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' means one lot of letters go next to another.
'half' means to take half the letters.
'brogue' halved 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" .)