Old person using their teeth in passing (6)
I believe the answer is:
obiter
'in passing' is the definition.
(legal term meaning said in passing)
'old person using their teeth' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'person using their teeth' becomes 'biter' (someone who bites).
'o'+'biter'='OBITER'
(Other definitions for obiter that I've seen before include "By the way (Latin)" , "..... dictum means any incidental remark" , "In passing" , "Incidental remark in Latin - . . . . . . dictum" , "incidentally" .)