To be a saint is superlatively good (4)
I believe the answer is:
best
'superlatively good' is the definition.
(I've seen this before)
'to be a saint' is the wordplay.
'a saint' becomes 'st'.
'be'+'st'='BEST'
'is' acts as a link.
(Other definitions for best that I've seen before include "or just the opposite?" , "Beat" , "Overcome" , "BOSOM] ?" , "former football star" .)