Sage mixture gets old (4)
I believe the answer is:
ages
'gets old' is the definition.
('age' can be a synonym of 'get old')
'sage mixture' is the wordplay.
'mixture' indicates an anagram.
'sage' anagrammed gives 'AGES'.
(Other definitions for ages that I've seen before include "Sage (anag)" , "Yonks" , "... of Man, according to WB Yeats" , "Gets older" , "Matures" .)