Astonish tabloid, seizing end of report (4)
I believe the answer is:
stun
'astonish' is the definition.
(I've seen this before)
'tabloid seizing end of report' is the wordplay.
'tabloid' becomes 'Sun' (The Sun, UK newspaper).
'seizing' is an insertion indicator (inserted letters are seized).
'end of' says to take the final letters.
The last letter of 'report' is 't'.
'sun' enclosing 't' is 'STUN'.
(Other definitions for stun that I've seen before include "Astound; render senseless" , "Shock into silence" , "Knock cold" , "Render unconscious" , "deliver a knock-out blow" .)