Snakes always poisonous? Extremely! (4)
I believe the answer is:
asps
'snakes' is the definition.
(asp is a kind of snake)
'always poisonous? extremely' is the wordplay.
'always' becomes 'as' (I can't justify this - if you can you should believe this answer much more).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'poisonous' with its middle removed is 'ps'.
'as'+'ps'='ASPS'
Can you help me to learn more?
(Other definitions for asps that I've seen before include "native Africans?" , "Cleopatra's snakes" , "Poisonous snakes" , "Small venomous snakes" , "poisonous things" .)