Most rational form of assent (6)
I believe the answer is:
sanest
'most rational' is the definition.
('sane' is associated in meaning with 'rational')
'form of assent' is the wordplay.
'form of' is an anagram indicator (the letters need to be in another 'form').
'assent' with letters rearranged gives 'SANEST'.
(Other definitions for sanest that I've seen before include "Most mentally well" , "Mentally most healthy" , "Most reasoned and mentally sound" , "Assent, being most rational" , "Most sensible" .)