Poisonous arachnid (8)
I believe the answer is:
scorpion
'poisonous arachnid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scorpion that I've seen before include "Small spider with pincers and stinging tail" , "Arachnid with venomous sting" , "Eight sign of Zodiac, kind of spider" , "Poisonous arachnid" , "Stinging creature" .)