Cultivating sage for years (4)
I believe the answer is:
ages
'for years' is the definition.
(I've seen this before)
'cultivating sage' is the wordplay.
'cultivating' indicates an anagram.
'sage' with letters rearranged gives 'AGES'.
(Other definitions for ages that I've seen before include "Historic periods or prolonged time" , "Grows old like sage" , "Sage (anag)" , "Yonks" , "Long periods" .)