Fishy product occupies schooner, say (9)
I believe the answer is:
isinglass
'fishy product' is the definition.
(gelatin from fish)
'occupies schooner say' is the wordplay.
'occupies' becomes 'is in' (to occupy something is to be in it).
'schooner say' becomes 'glass' (schooner is a type of glass).
'isin'+'glass'='ISINGLASS'
(Other definitions for isinglass that I've seen before include "Lass, I sing badly about this preservative" , "Gelatinous substance from sturgeons" , "What's got from fish" , "Fish product used in brewing" , "fish produce" .)