Almost certainly genuine, but bizarre (7)
I believe the answer is:
surreal
'bizarre' is the definition.
(I've seen this before)
'almost certainly genuine' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'genuine' becomes 'real' ('real' is associated in meaning with 'genuine').
'sure' with its last letter taken away is 'sur'.
'sur'+'real'='SURREAL'
'but' acts as a link.
(Other definitions for surreal that I've seen before include "Daliesque" , "Like Dada image, perhaps" , "Like a dream" , "Weird - like Dali's paintings?" , "Like the paintings of Rene Magritte" .)