Decay beginning to crack a sign (6)
I believe the answer is:
caries
'decay' is the definition.
(caries is a kind of decay)
'beginning to crack a sign' is the wordplay.
'beginning to' indicates taking the first letters.
'a sign' becomes 'aries' (Aries is an example).
The first letter of 'crack' is 'c'.
'c'+'aries'='CARIES'
(Other definitions for caries that I've seen before include "Decayed areas in teeth" , "Dental decay" , "Dental cavities" , "Decay of bone or teeth" , "Decay in bones or teeth (6)" .)