So urgent to cook fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'so urgent to cook' is the wordplay.
'to cook' indicates an anagram.
'so'+'urgent'='sourgent'
'sourgent' with letters rearranged gives 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "The fish that yields caviar" , "Fish - got nurse (anag)" , "Source of caviar" , "Fish providing isinglass" , "producer of expensive eggs" .)