Bad move to grab scorpion's tail: poison! (5)
I believe the answer is:
venom
'poison' is the definition.
(venom is a kind of poison)
'bad move to grab scorpion's tail' is the wordplay.
'bad' indicates anagramming the letters.
'to grab' indicates putting letters inside.
'tail' indicates one should take the final letters.
The last letter of 'scorpion' is 'n'.
'move' is an anagram of 'veom'.
'veom' going around 'n' is 'VENOM'.
(Other definitions for venom that I've seen before include "Bitter malice" , "Spite: toxin" , "essential part of The Sting" , "bitterness" , "Poisonous liquid of snakes" .)