Tooth decay -- constant sign (6)
I believe the answer is:
caries
'tooth decay' is the definition.
(term for decay of a tooth or bone)
'constant sign' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'sign' becomes 'Aries' (star sign).
'c'+'aries'='CARIES'
(Other definitions for caries that I've seen before include "Dental cavities" , "Dental decay" , "Decayed area in a tooth" , "Decayed areas in teeth" , "I scare easily with tooth decay" .)