Heads off awful snake, poisonous — this one? (3)
I believe the answer is:
asp
'this one?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'heads off awful snake poisonous' is the wordplay.
'heads off' suggests taking the first letters (I've seen 'head off' mean this).
The first letters of 'awful snake poisonous' is 'ASP'.
(Other definitions for asp that I've seen before include "Egyptian cobra" , "Predatory fish" , "Snake found in spa" , "Cleopatra's supposed nemesis" , "Small viper" .)