Cultivating sage takes a long time (4)
I believe the answer is:
ages
'a long time' is the definition.
(ages can mean a very long time)
'cultivating sage' is the wordplay.
'cultivating' indicates an anagram.
'sage' anagrammed gives 'AGES'.
'takes' is the link.
(Other definitions for ages that I've seen before include "Historical eras" , "Long periods" , "Epochs" , "Historic periods or prolonged time" , "Grows old like sage" .)