Most reasonable if rates are adjusted (7)
I believe the answer is:
fairest
'most reasonable' is the definition.
('fair' is associated in meaning with 'reasonable')
'if rates are adjusted' is the wordplay.
'are adjusted' indicates anagramming the letters.
'if'+'rates'='ifrates'
'ifrates' with letters rearranged gives 'FAIREST'.
(Other definitions for fairest that I've seen before include "Most beautiful" , "Very lovely" , "'''Mirror, mirror on the wall, who is the ... of them all'' (Snow White's stepmother) (7)'" .)