Giant imaginary creatures (8)
I believe the answer is:
monsters
'giant imaginary creatures' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for monsters that I've seen before include "Ugly, terrifying creatures" , "Imaginary frightening creatures" , "Giant imaginary creatures" , "Behemoths, fiends" , "'Imaginary creatures, large and wicked (8)'" .)