Sage cultivated for a long time (4)
I believe the answer is:
ages
'long time' is the definition.
(I've seen this before)
'sage cultivated' is the wordplay.
'cultivated' is an anagram indicator.
'sage' is an anagram of 'AGES'.
'for a' acts as a link.
(Other definitions for ages that I've seen before include "Epochs" , "Awfully long time" , "Yonks" , "Times" , "Historic periods or prolonged time" .)