Exert reforms this person trails as radical (7)
I believe the answer is:
Here is my best explanation:
'radical' is the definition.
'exert reforms this person trails' is the subsidiary indication.
'reforms' indicates anagramming the letters.
'this person' becomes 'me' (I've seen this in other clues).
'trails' says to put letters next to each other (some letters follow or trail others).
'exert' is an anagram of 'extre'.
'as' is the link.
(Other definitions for extreme that I've seen before include "Ultimate", "Great", "far out", "Far from centre", "Outermost", "Very great; exceptional", "Radical", "immoderate", "over the top" and "Most remote or utmost".)