Not that bit of ancient history (4)
I believe the answer is:
this
'not that' is the definition.
(I've seen this before)
'bit of ancient history' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'THIS' is hidden within 'ancient history'.
(Other definitions for this that I've seen before include "The thing here referred to" , "Hits (anag.)" , "One particular" , "The thing just mentioned" , "What's here" .)