First sign of tooth decay? Not the first (5)
I believe the answer is:
aries
'first sign' is the definition.
(I've seen this in another clue)
'tooth decay? not the first' is the wordplay.
'tooth decay?' becomes 'caries' (dental caries).
'not the first' means to remove the first letter.
'caries' with its initial letter removed is 'ARIES'.
'of' is the link.
(Other definitions for aries that I've seen before include "Zodiacal sign also known as the Ram" , "Philip -, English poet" , "star group" , "A sign from Heaven" , "Sheep" .)