Shock as spectacular feat is left unfinished (4)
I believe the answer is:
stun
'shock' is the definition.
(synonyms)
'spectacular feat is left unfinished' is the wordplay.
'spectacular feat' becomes 'stunt'.
'is left unfinished' means to remove the last letter.
'stunt' with its last letter taken away is 'STUN'.
'as' is the link.
(Other definitions for stun that I've seen before include "Daze with nuts" , "Render unconscious" , "Dumbfound" , "Surprise greatly or daze" , "Nuts (anag)" .)