Foam from venomous serpent (6)
I believe the answer is:
mousse
'foam' is the definition.
(I know that mousse is a type of foam)
'from venomous serpent' is the wordplay.
'from' indicates the answer is hidden within the clue.
'MOUSSE' is hidden within 'venomous serpent'.
(Other definitions for mousse that I've seen before include "Frothy creamy substance for styling hair" , "Shock treatment" , "Whipped savoury dish" , "Light creamy dessert, set with gelatin" , "Whipped sweet" .)