Happening by chance (10)
'happening by chance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for accidental that I've seen before include "Happening by chance" , "without planning" , "not intended by composer?" , "change of key" , "Sign raising or lowering a musical note" .)
(Other definitions for contingent that I've seen before include "Depending (on) - part of military force" , "army unit" , "Company" , "Quota of troops" , "group of people" .)