Time of underachievement (3)
I believe the answer is:
era
'time' is the definition.
(an era is a period of time)
'of underachievement' is the wordplay.
'of' says the answer is hidden in the clue.
'ERA' can be found hidden inside 'underachievement'.
(Other definitions for era that I've seen before include "Memorable period" , "generation, maybe" , "A particular period of time" , "Long historical period" , "Are involved in time span" .)