It's poisonous, so you need less than a teaspoonful (3)
I believe the answer is:
asp
'it's poisonous so' is the definition.
I can't judge whether this defines the answer.
'less than a teaspoonful' is the wordplay.
'less' indicates the answer is hidden within the clue (the clue less some of the letters in it).
'ASP' is hidden in the letters of 'than teaspoonful'.
'you need' acts as a link.
Can you help me to learn more?
(Other definitions for asp that I've seen before include "It proved deadly for Cleopatra" , "the snake in Cleopatra''s bosom" , "Cleopatra's poisoner" , "This Egyptian killer" , "Snake of the Nile" .)