Herb lives with King Serpent (8)
I believe the answer is:
basilisk
'serpent' is the definition.
(I have seen 'Mythical serpent ' mean 'basilisk' so perhaps 'serpent' could also mean 'basilisk')
'herb lives with king' is the wordplay.
'herb' becomes 'basil' (basil is a herb).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'with' means one lot of letters go next to another.
'king' becomes 'K' (abbreviation for king in chess).
'basil'+'is'+'k'='BASILISK'
(Other definitions for basilisk that I've seen before include "Lethal snake of mythology" , "Mythical beast" , "Mythical reptile with a lethal gaze" , "Mythical reptile with deadly glance and breath" , "Cockatrice" .)