Past good deal a mistake (9)
I believe the answer is:
oversight
'a mistake' is the definition.
(oversight is a kind of mistake)
'past good deal' is the wordplay.
'past' becomes 'over' (I've seen this before).
'good deal' becomes 'sight' ('sight' can be a synonym of 'good deal').
'over'+'sight'='OVERSIGHT'
(Other definitions for oversight that I've seen before include "Mistake resulting from inattention" , "act of negligence" , "Neglect" , "Failure to notice something" , "Innocent mistake" .)