Radical old meter is used (7)
I believe the answer is:
extreme
'radical' is the definition.
(radical can mean revolutionary or extreme)
'old meter is used' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'is used' indicates anagramming the letters.
'meter' is an anagram of 'treme'.
'ex'+'treme'='EXTREME'
(Other definitions for extreme that I've seen before include "end" , "Exceptional - radical" , "Most distant" , "Farthest - to the highest degree" , "Greatest or smallest" .)