Little brother arrived in time (3)
I believe the answer is:
'time' is the definition.
(era is a kind of time)
'little brother arrived in' is the wordplay.
'in' indicates a hidden word.
'ERA' is hidden within 'little brother arrived'.
(Other definitions for era that I've seen before include "Important date" , "Characteristic period" , "Long historical period" , "day" , "Period, age" .)