Imaginary monster (8)
I believe the answer is:
bogeyman
'imaginary monster' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bogeyman that I've seen before include "Children fear him - a bad golfer?" , "Imaginary monster to frighten children" , "Monster used to frighten children" , "Spectre terrifying to children" , "Person (real or imaginary) used as a threat to children" .)