Historical zenith (6,3)
I believe the answer is:
golden age
'historical zenith' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for golden age that I've seen before include "Most flourishing and outstanding period" , "A time of great prosperity - 24 carats?" , "Idyllic past time" , "Time when things were at their best" , "Imaginary idyllic period in the past" .)