The age in which Chaucer achieved fame (3)
I believe the answer is:
era
'the age' is the definition.
(both can mean a long time)
'in which chaucer achieved fame' is the wordplay.
'in which' indicates a hidden word.
'ERA' is hidden within 'chaucer achieved fame'.
(Other definitions for era that I've seen before include "Cycle" , "Period, age" , "Important historical period" , "Specific time" , "Important date" .)