Back a great deal, showing wisdom after the event (9)
I believe the answer is:
hindsight
'wisdom after the event' is the definition.
(I've seen this before)
'back a great deal' is the wordplay.
'back' becomes 'hind' ('hind' can be a synonym of 'back').
'a great deal' becomes 'sight' ('sight' can be a synonym of 'great deal'**).
'hind'+'sight'='HINDSIGHT'
'showing' acts as a link.
(Other definitions for hindsight that I've seen before include "Ken, after event" , "review" , "Thinking back" , "Looking back" , "Retrospection" .)