Radical former lover getting married in plane? (7)
I believe the answer is:
'radical' is the definition.
(radical can mean revolutionary or extreme)
'former lover getting married in plane?' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'getting' means one lot of letters go next to another.
'married' becomes 'm'.
'in' is an insertion indicator.
'plane?' becomes 'tree' (plane tree is a type of tree).
'm' placed within 'tree' is 'treme'.
(Other definitions for extreme that I've seen before include "Most violent" , "Going beyond moderation" , "Going to great lengths" , "sheer" , "Last" .)