Breathtaking time in afternoon, going off (10)
I believe the answer is:
astounding
'breathtaking' is the definition.
(thesaurus)
'time in afternoon going off' is the wordplay.
'time' becomes 't' (abbreviation).
'in' is an insertion indicator.
'afternoon' becomes 'a' (abbreviation).
'going off' becomes 'sounding' (eg an alarm clock sounding or going off).
'a'+'sounding'='asounding'
't' inserted within 'asounding' is 'ASTOUNDING'.
(Other definitions for astounding that I've seen before include "Extraordinary" , "Stunning" , "Amazing" .)