Fish in British stream (5)
I believe the answer is:
brill
'fish' is the definition.
(brill is a kind of fish)
'british stream' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'stream' becomes 'rill' (rill is a kind of stream).
'b'+'rill'='BRILL'
'in' acts as a link.
(Other definitions for brill that I've seen before include "Excellent (abbr) - fish" , "catch, perhaps" , "Very good - fish" , "Flatfish; excellent" , "marvellous" .)