Time of underachieving (3)
I believe the answer is:
era
'time' is the definition.
(era is a kind of time)
'of underachieving' is the wordplay.
'of' says the answer is hidden in the clue.
'ERA' can be found hidden inside 'underachieving'.
(Other definitions for era that I've seen before include "Epoch, period" , "Are involved in time span" , "quite some time" , "Period, age" , "Characteristic period" .)