Interim ruler holding on to France, mounting a comeback (9)
I believe the answer is:
resurgent
'mounting a comeback' is the definition.
(resurgent means reviving or coming back)
'interim ruler holding on to france' is the wordplay.
'interim ruler' becomes 'regent' (a regent rules on behalf of a monarch).
'holding' means one lot of letters goes inside another.
'on to france' becomes 'sur' ('on' in French).
'regent' placed around 'sur' is 'RESURGENT'.
(Other definitions for resurgent that I've seen before include "Reviving after period of inactivity" , "making comeback" , "rising again" , "given new life" , "Reviving after a lull" .)