That's what you think! (7)
'that's what you think' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opinion that I've seen before include "Personal belief or judgment" , "Belief not founded on proof or certainty" , "Unproved belief or view" , "Belief or conviction" , "View, judgement" .)
(Another definition for hah that I've seen is " Expression of surprise (one letter short of a laugh)".)