Exceptional Frenchman in old tree (7)
I believe the answer is:
extreme
'exceptional' is the definition.
(synonyms)
'frenchman in old tree' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'in' indicates putting letters inside.
'old' becomes 'ex' (prefix meaning former or onetime).
'ex'+'tree'='extree'
'm' put into 'extree' is 'EXTREME'.
(Other definitions for extreme that I've seen before include "Acute, maximum" , "Far from centre" , "Ultimate" , "Unrestrained" , "Severe; radical" .)