Shut up? About time! (4)
I believe the answer is:
cage
'shut up?' is the definition.
(thesaurus)
'about time' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'time' becomes 'age' (age is a kind of time**).
'c'+'age'='CAGE'
(Other definitions for cage that I've seen before include "Barred accommodation for animal" , "Place of confinement" , "Prison for 21" , "Tffhe goal in ice hockey" , "Confine -- US composer, d. 1992" .)