Chance money originally coming in, to do with teeth (12)
I believe the answer is:
coincidental
'chance' is the definition.
(as in a chance encounter)
'money originally coming in to do with teeth' is the wordplay.
'money' becomes 'coin' (coin is a type of money).
'originally' indicates taking the first letters.
'to do with teeth' becomes 'dental' (as in dentistry).
The initial letters of 'coming in' is 'ci'.
'coin'+'ci'+'dental'='COINCIDENTAL'
(Other definitions for coincidental that I've seen before include "Occurring together" , "Apparently randomly concurrent" , "Simultaneous" , "Happening by chance" , "fortuitous" .)