Chance event that's grave, perhaps concealing identity (8)
I believe the answer is:
accident
'chance event' is the definition.
(synonyms)
'grave perhaps concealing identity' is the wordplay.
'grave perhaps' becomes 'accent' (grave accent is a type of accent mark).
'concealing' is an insertion indicator.
'identity' becomes 'id' (abbreviation).
'accent' going around 'id' is 'ACCIDENT'.
'that's' is the link.
(Other definitions for accident that I've seen before include "Unexpected and unplanned event" , "Unfortunate unintentional incident" , "Contretemps" , "Prang" , "Unexpected incident" .)