Very long time taken in Cannae once (4)
I believe the answer is:
'very long time' is the definition.
(an aeon or eon is a long time)
'taken in cannae once' is the wordplay.
'taken in' says the answer is hidden in the clue.
'AEON' can be found hidden inside 'cannae once'.
(Other definitions for aeon that I've seen before include "Indefinitely long period of time" , "long term" , "A very long time, an age" , "Immeasurably long period of time" , "Eternity" .)