One-eyed monster (7)
I believe the answer is:
cyclops
'one-eyed monster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cyclops that I've seen before include "one with limited perception" , "Electronic tennis aid" , "The mythical giant with a single eye in the middle of his forehead" , "One-eyed monster" , "Mythical one-eyed giant" .)