Worries about start of incisor decay (6)
I believe the answer is:
caries
'decay' is the definition.
(dental caries)
'worries about start of incisor' is the wordplay.
'worries' becomes 'cares' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'start of' suggests taking the first letters.
The initial letter of 'incisor' is 'i'.
'cares' going around 'i' is 'CARIES'.
(Other definitions for caries that I've seen before include "Decayed areas in teeth" , "I scare easily with tooth decay" , "Decay in bones or teeth (6)" , "Decayed area in a tooth" , "Dental decay" .)