Great whites head flounder off (9)
I believe the answer is:
wonderful
'great' is the definition.
(both can mean amazing or grand)
'whites head flounder off' is the wordplay.
'head' indicates taking the first letters.
'off' indicates an anagram.
The initial letter of 'whites' is 'w'.
'flounder' anagrammed gives 'onderful'.
'w'+'onderful'='WONDERFUL'
(Other definitions for wonderful that I've seen before include "Superb" , "Great" , "Awe-inspiring" , "Fab" , "Excellent" .)