Overassertive about times (4)
I believe the answer is:
eras
'times' is the definition.
(era is a kind of time)
'overassertive about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'ERAS' is hidden in the letters of 'overassertive'.
(Other definitions for eras that I've seen before include "At least two times" , "Periods, ages" , "typeface" , "Periods of time" , "Epochs" .)