Radical triad's oddly coy leader (7)
I believe the answer is:
drastic
'radical' is the definition.
(drastic can mean radical or extreme)
'triad's oddly coy leader' is the wordplay.
'oddly' indicates an anagram.
'leader' suggests taking the first letters.
The first letter of 'coy' is 'c'.
'triads' is an anagram of 'drasti'.
'drasti'+'c'='DRASTIC'
(Other definitions for drastic that I've seen before include "Uncompromising" , "Grave" , "Thoroughgoing" , "'Severe, extreme (7)'" , "Unsparing" .)