Most frightening way to arrest tooth decay (8)
I believe the answer is:
scariest
'most frightening' is the definition.
(the most scary)
'way to arrest tooth decay' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to arrest' indicates putting letters inside (inserted letters are arrested or held).
'tooth decay' becomes 'caries' (term for decay of a tooth or bone).
'st' going around 'caries' is 'SCARIEST'.
(Other definitions for scariest that I've seen before include "Most frightening" , "most alarming" , "Most terrifying" .)