Radical Frenchman in old tree (7)
I believe the answer is:
extreme
'radical' is the definition.
(radical can mean revolutionary or extreme)
'frenchman in old tree' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'in' means one lot of letters goes inside another.
'old' becomes 'ex' (prefix meaning former or onetime).
'ex'+'tree'='extree'
'm' put within 'extree' is 'EXTREME'.
(Other definitions for extreme that I've seen before include "Severe; radical" , "stiff" , "Farthest - to the highest degree" , "Draconian" , "very strict" .)