Upset move to seize assassin's last poison (5)
I believe the answer is:
venom
'poison' is the definition.
(venom is a kind of poison)
'upset move to seize assassin's last' is the wordplay.
'upset' is an anagram indicator.
'to seize' means one lot of letters goes inside another.
'last' suggests the final letters.
The last letter of 'assassin' is 'n'.
'move' anagrammed gives 'veom'.
'veom' placed around 'n' is 'VENOM'.
(Other definitions for venom that I've seen before include "bitterness" , "Virulence" , "Poison; malice" , "Snake poison" , "Poisonous secretion" .)