Unbelievable (10)
'unbelievable' is the definition.
(I know that unbelievable can be written as incredible)
This is the entire clue.
(Other definitions for incredible that I've seen before include "Unbelievable" , "Fantastic" , "Beyond belief" , "Impossible to believe" .)
(Other definitions for i say that I've seen before include "Good heavens!" , "Look here; oh, my" , "Comment from protestor" , "Dear me!" , "Crikey!" .)