Most worrying tooth decay, in a way (8)
I believe the answer is:
scariest
'most worrying' is the definition.
(scary can mean worrying or concerning)
'tooth decay in a way' is the wordplay.
'tooth decay' becomes 'caries' (term for decay of a tooth or bone).
'in' is an insertion indicator.
'a way' becomes 'st'.
'caries' put inside 'st' is 'SCARIEST'.
(Other definitions for scariest that I've seen before include "Most frightening" , "Most terrifying" , "most alarming" .)