Age Fitzgerald's written about (3)
I believe the answer is:
era
'age' is the definition.
(both can mean a long time)
'fitzgerald's written about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'ERA' is hidden in the letters of 'fitzgeralds written'.
(Other definitions for era that I've seen before include "Significant time" , "Aeon" , "Characteristic period" , "Long historic period" , "Important date" .)