Roughly, a sign of tooth decay (6)
I believe the answer is:
caries
'tooth decay' is the definition.
('caries' can be a synonym of 'tooth decay')
'roughly a sign' is the wordplay.
'roughly' becomes 'c' (circa).
'a sign' becomes 'aries' (star sign).
'c'+'aries'='CARIES'
'of' acts as a link.
(Other definitions for caries that I've seen before include "Decayed areas in teeth" , "Dental decay" , "I scare easily with tooth decay" , "Decay in bones or teeth (6)" , "Decay of bone or teeth" .)