Exert reforms this person trails as radical (7)
I believe the answer is:
extreme
'radical' is the definition.
(thesaurus)
'exert reforms this person trails' is the wordplay.
'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' anagrammed gives 'extre'.
'extre'+'me'='EXTREME'
'as' is the link.
(Other definitions for extreme that I've seen before include "too much" , "Inordinate" , "Farthest - to the highest degree" , "Ultimate" , "Immoderate" .)