Ripens; a long time (4)
I believe the answer is:
ages
I believe this is a double definition.
'ripens' is the first definition.
(I've seen this in another clue)
'a long time' is the second definition.
(ages can mean a very long time)
(Other definitions for ages that I've seen before include "Gets older" , "A long time (colloq.)" , "Epochs" , "Donkey's years" , "Matures" .)