Assent to reform most reasonable (6)
I believe the answer is:
sanest
'most reasonable' is the definition.
('sane' can be similar in meaning to 'reasonable')
'assent to reform' is the wordplay.
'to reform' indicates anagramming the letters (letters in a new form).
'assent' is an anagram of 'SANEST'.
(Other definitions for sanest that I've seen before include "Most reasoned and mentally sound" , "Assent, being most rational" , "Most rational and reasonable" , "Mentally most healthy" , "Most rational and logical" .)