Monster — huge thing (8)
I believe the answer is:
behemoth
This clue is a double definition.
'monster' is the first definition.
(synonyms)
'huge thing' is the second definition.
(I've seen this in another clue)
(Other definitions for behemoth that I've seen before include "Something huge and monstrous" , "Anything abnormally large and powerful" , "Abnormally large creature" , "Something abnormally large and powerful" , "Something of impressive size or power" .)