Fish cook mostly served eagerly at first (6)
I believe the answer is:
grilse
'fish' is the definition.
(I have seen 'Young fish' mean 'grilse' so perhaps 'fish' could also mean 'grilse')
I don't understand how the remainder of the clue works.
(Other definitions for grilse that I've seen before include "Young salmon returning to fresh water for the first time" , "Young fish" , "Young salmon (6)" , "Young salmon returning for first spawning" , "A one-year old salmon" .)