Age’s back-page planned (3)
I believe the answer is:
eon
'age's' is the definition.
(eon is a kind of age)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eon that I've seen before include "billion years" , "Era" , "Immense period" , "Vast period" , "thousand million years" .)