Radical former lover married inside plane, perhaps (7)
I believe the answer is:
extreme
'radical' is the definition.
(thesaurus)
'former lover married inside plane perhaps' is the wordplay.
'former lover' becomes 'ex'.
'married' becomes 'm'.
'inside' means one lot of letters goes inside another.
'plane perhaps' becomes 'tree' (I've seen this in other clues).
'm' going within 'tree' is 'treme'.
'ex'+'treme'='EXTREME'
(Other definitions for extreme that I've seen before include "lacking moderation" , "Draconian" , "Acute, maximum" , "Inordinate" , "Last" .)