Radical Mike in old tree (7)
I believe the answer is:
extreme
'radical' is the definition.
(radical can mean revolutionary or extreme)
'mike in old tree' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'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 "Most violent" , "very strict" , "furthest from the centre" , "Last" , "Very great; exceptional" .)