Idiot grabbing first of poisonous snakes (4)
I believe the answer is:
asps
'snakes' is the definition.
(asp is a kind of snake)
'idiot grabbing first of poisonous' is the wordplay.
'idiot' becomes 'ass' (I've seen this before).
'grabbing' means one lot of letters goes inside another.
'first of' indicates taking the first letters.
The initial letter of 'poisonous' is 'p'.
'ass' placed around 'p' is 'ASPS'.
(Other definitions for asps that I've seen before include "poisonous things" , "native Africans?" , "people" , "Pass around the snakes" , "because of their venom?" .)