Snake about to bite Rob to bits (5)
I believe the answer is:
cobra
'snake' is the definition.
(I know that snake can be written as Cobra)
'about to bite rob to bits' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to bite' means one lot of letters goes inside another (to have a bite can mean to consume).
'to bits' is an anagram indicator.
'rob' anagrammed gives 'obr'.
'ca' going around 'obr' is 'COBRA'.
(Other definitions for cobra that I've seen before include "Poisonous snake" , "dangerous striker" , "crisis response group" , "Hooded snake - AC model of old sports car" , "Something with a hood" .)