About time to shut up (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'
'to' acts as a link.
(Other definitions for cage that I've seen before include "Imprison, confine" , "Composer working in silence" , "he used to write down scores" , "Put in prison" , "John ---, composer; one with bars" .)