Serpent that has poison inside (3)
I believe the answer is:
asp
'serpent' is the definition.
(asp is a kind of serpent)
'that has poison inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'ASP' is hidden in the letters of 'that has poison'.
(Other definitions for asp that I've seen before include "Cleopatra's supposed nemesis" , "Kind of snake - what a sap" , "Pa's Cleopatra's killer" , "Predatory fish" , "Snake, killed Cleopatra" .)