Long journey back in darker times (4)
I believe the answer is:
trek
'long journey' is the definition.
(I've seen this before)
'back in darker times' is the wordplay.
'back' is a reversal indicator.
'in' indicates a hidden word.
'kert' can be found hidden inside 'darker times'.
'kert' backwards is 'TREK'.
(Other definitions for trek that I've seen before include "Long hard journey on foot" , "Difficult journey on foot" , "Tramp" , "A long hard march" , "Long walk in the mountains?" .)