Saint about dental decay that is most alarming (8)
I believe the answer is:
scariest
'is most alarming' is the definition.
('scary' can be similar in meaning to 'alarming')
'saint about dental decay' is the wordplay.
'saint' becomes 'st'.
'about' indicates putting letters inside.
'dental decay' becomes 'caries' (I've seen this before).
'st' enclosing 'caries' is 'SCARIEST'.
'that' acts as a link.
(Other definitions for scariest that I've seen before include "most alarming" , "Most frightening" , "Most terrifying" .)