Some of the higher ascents take ages! (4)
I believe the answer is:
eras
'ages' is the definition.
(I've seen this before)
'some of the higher ascents take' is the wordplay.
'some of' says the answer is hidden in the clue.
'ERAS' is hidden in the letters of 'higher ascents take'.
(Other definitions for eras that I've seen before include "At least two times" , "Periods, ages" , "Epochs" , "Long and distinct periods of history" , "typeface" .)