Most sensible, learned saint (6)
I believe the answer is:
wisest
'most sensible' is the definition.
(I've seen this before)
'learned saint' is the wordplay.
'learned' becomes 'wise' (I've seen this before).
'saint' becomes 'st' (abbreviation).
'wise'+'st'='WISEST'
(Other definitions for wisest that I've seen before include "Shrewdest" , "Superlatively sage" , "Most sensible" , "Most sage and sagacious" , "Most sage and learned" .)