A long time to cultivate sage (4)
I believe the answer is:
ages
'a long time' is the definition.
(ages can mean a very long time)
'to cultivate sage' is the wordplay.
'to cultivate' indicates an anagram (I've seen 'cultivated' mean this).
'sage' anagrammed gives 'AGES'.
(Other definitions for ages that I've seen before include "Rock of - - (hymn)" , "Epochs" , "Awfully long time" , "Gets older" , "Matures" .)