Dangerous snake (5)
(Other definitions for cobra that I've seen before include "hamadryad" , "a king, for example" , "One with hood" , "Type of snake" , "Reptile (found in Cabinet Office Briefing Room A?)" .)
'dangerous snake' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for adder that I've seen before include "on country walks?" , "He's making an extension" , "Type of snake" , "Venomous sort" , "venomous creature" .)
(Other definitions for mamba that I've seen before include "serpent" , "one venomous" , "One's venomous" , "Poisonous creature" , "Snake that might kill" .)