Insect losing head sees deadly snake (3)
I believe the answer is:
asp
'deadly snake' is the definition.
(I've seen this in another clue)
'insect losing head' is the wordplay.
'insect' becomes 'wasp' (wasp is a kind of insect).
'losing head' means to remove the first letter.
'wasp' with its initial letter removed is 'ASP'.
'sees' acts as a link.
(Other definitions for asp that I've seen before include "Pa's Cleopatra's killer" , "Predatory fish" , "Joe Blake is Australian species" , "something at the zoo" , "This Egyptian killer" .)