Astonish tabloid, seizing end of report (4)
I believe the answer is:
stun
'astonish' is the definition.
(I've seen this in another clue)
'tabloid seizing end of report' is the wordplay.
'tabloid' becomes 'Sun' (The Sun, UK newspaper).
'seizing' indicates putting letters inside (inserted letters are seized).
'end of' indicates one should 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" .)