Years in a stage success (4)
I believe the answer is:
ages
'years' is the definition.
(I know this)
'in a stage success' is the wordplay.
'in' says the answer is hidden in the clue.
'AGES' can be found hidden inside 'stage success'.
(Other definitions for ages that I've seen before include "Grows old like sage" , "Indefinitely long time" , "Rock of - - (hymn)" , "Sage (anag)" , "Historic periods or prolonged time" .)