Tooth decay — one probing is concerned (6)
I believe the answer is:
caries
'tooth decay' is the definition.
(synonyms)
'one probing is concerned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'is concerned' becomes 'cares' (to care is show concern or worry).
'i' placed inside 'cares' is 'CARIES'.
(Other definitions for caries that I've seen before include "Decayed areas in teeth" , "Dental cavities" , "Decay of bone or teeth" , "Decay in bones or teeth (6)" , "Dental decay" .)