A not-too-serious sin (6)
I believe the answer is:
venial
'a not-too-serious sin' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for venial that I've seen before include "A not-too-serious sin" , "Excusable" , "offering excuse" , "Slight and pardonable (sin)" , "An evil (anag) - easily excused" .)