Times long ago opposing Leave? Yes, at first (9)
I believe the answer is:
antiquity
'times long ago' is the definition.
'opposing leave? yes at first' is the wordplay.
'opposing' becomes 'anti'.
'leave?' becomes 'quit' (quitting is a kind of leaving**).
'at first' says to take the initial letters.
The initial letter of 'yes' is 'y'.
'anti'+'quit'+'y'='ANTIQUITY'
(Other definitions for antiquity that I've seen before include "days of yore" , "Ancient past" , "The far distant past" , "Olden days" , "Time immemorial" .)