Hardy variety of monster (5)
I believe the answer is:
hydra
'monster' is the definition.
(I've seen this before)
'hardy variety' is the wordplay.
'variety' is an anagram indicator.
'hardy' with letters rearranged gives 'HYDRA'.
'of' is the link.
(Other definitions for hydra that I've seen before include "Water monster" , "Proliferating-head monster" , "Mythological monster with nine heads" , "Mythical many-headed snake" , "Multi-headed monster slain by Hercules" .)