Weapon seizure upset to some extent (3)
I believe the answer is:
uzi
'weapon' is the definition.
(I've seen this before)
'seizure upset to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'upset' says the letters should be written in reverse.
'izu' is hidden within 'seizure'.
'izu' reversed gives 'UZI'.
(Other definitions for uzi that I've seen before include "killer" , "kind of weapon" , "Submachine-gun" , "Type of submachine gun" , "Israeli-designed sub-machine gun" .)