Sign of tooth decay? Not the first (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'tooth decay? not the first' is the wordplay.
'tooth decay?' becomes 'caries' ('caries' can be a synonym of 'tooth decay').
'not the first' means to remove the first letter.
'caries' with its first letter taken off is 'ARIES'.
'of' is the link.
(Other definitions for aries that I've seen before include "A sign from Heaven" , "Ram star sign" , "star group" , "Stars" , "Zodiacal sign also known as the Ram" .)