Fish that is grand, superior to two others (8)
I believe the answer is:
grayling
'fish that is grand superior to two others' is the definition.
'grayling' can be an answer for 'fish' (I've seen this before). I am not certain of the remainder of the definition.
'fish that is grand superior to two' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'that is grand' becomes 'G' (G is short for ground as in a thousand).
'superior to' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'two' becomes 'ling' (I can't justify this - if you can you should believe this answer much more).
'ray' put after 'g' is 'gray'.
'gray'+'ling'='GRAYLING'
(Other definitions for grayling that I've seen before include "Salmonoid fish" , "Fish of the salmon family" , "Catch" , "Butterfly" , "Fish - philosopher" .)