Starts to talk up river — bags of tasty fish! (6)
I believe the answer is:
turbot
'fish' is the definition.
(turbot is a kind of fish)
'starts to talk up river bags of tasty' is the wordplay.
'starts to' suggests taking the first letters.
The first letters of 'talk up river bags of tasty' is 'TURBOT'.
(Other definitions for turbot that I've seen before include "Type of fish" , "Pale brown, speckled flatfish" , "Large brownish European flatfish" , "Flatfish - trout B (anag)" , "Species of flatfish" .)