Welsh, say, united to stop Frenchman (7)
I believe the answer is:
renegue
'welsh' is the definition.
(I've seen this before)
'say united to stop frenchman' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'united' becomes 'U' (as in Man U, English football team).
'to stop' is an insertion indicator (stop can mean to plug a hole in something).
'frenchman' becomes 'rene' (common French name).
'eg'+'u'='egu'
'egu' inserted inside 'rene' is 'RENEGUE'.
(Other definitions for renegue that I've seen before include "Break one's word" , "Fall to fulfil promise" , "Deny" , "Fail to fulfil a promise" , "Revoke at cards" .)