Amaze star -- about time! (4)
I believe the answer is:
stun
'amaze' is the definition.
(to stun is to amaze or shock)
'star about time' is the wordplay.
'star' becomes 'sun' (Sun is an example).
'about' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'sun' placed around 't' is 'STUN'.
(Other definitions for stun that I've seen before include "Leave speechless" , "Anaesthetise" , "Numb with nuts" , "Make dazed or numb" , "floor" .)