A French earl, terribly bizarre (6)
I believe the answer is:
unreal
'bizarre' is the definition.
(I've seen this in another clue)
'a french earl terribly' is the wordplay.
'a french' becomes 'un' ('a' in French).
'terribly' is an anagram indicator.
'earl' with letters rearranged gives 'real'.
'un'+'real'='UNREAL'
(Other definitions for unreal that I've seen before include "Not actually existing" , "Illusory, imaginary" , "Fanciful" , "Illusory, not actual" , "Not genuine" .)