Monster herb is leek in the end (8)
I believe the answer is:
basilisk
'monster' is the definition.
(basilisk is a kind of monster)
'herb is leek in the end' is the wordplay.
'herb' becomes 'basil' (basil is a herb).
'in the end' indicates one should take the final letters.
The last letter of 'leek' is 'k'.
'basil'+'is'+'k'='BASILISK'
(Other definitions for basilisk that I've seen before include "Mythical reptile with deadly glance and breath" , "Mythical monster" , "Small lizard of tropical America" , "fearful creature" , "By the look of it, killer" .)