Sage cultivated for yonks (4)
I believe the answer is:
ages
'yonks' is the definition.
(I've seen this before)
'sage cultivated' is the wordplay.
'cultivated' indicates anagramming the letters.
'sage' with letters rearranged gives 'AGES'.
'for' is the link.
(Other definitions for ages that I've seen before include "Rock of - - (hymn)" , "Aeons" , "A long time (colloq.)" , "Long periods" , "Epochs" .)