Optical illusion created by image moving over river (6)
I believe the answer is:
mirage
'optical illusion' is the definition.
(mirage is a kind of optical illusion)
'image moving over river' is the wordplay.
'moving' indicates an anagram.
'over' indicates putting letters inside.
'river' becomes 'r'.
'image' anagrammed gives 'miage'.
'miage' enclosing 'r' is 'MIRAGE'.
'created by' is the link.
(Other definitions for mirage that I've seen before include "fantasy" , "unreal" , "See it to believe it?" , "fighter craft" , "vision" .)