Chance money originally coming in, to do with teeth (12)
I believe the answer is:
'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'.
(Other definitions for coincidental that I've seen before include "Occurring together" , "Apparently randomly concurrent" , "Simultaneous" , "Happening by chance" , "fortuitous" .)