British river bad for fish (5)
I believe the answer is:
brill
'for fish' is the definition.
(brill is a kind of fish)
'british river bad' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'river' becomes 'r' (abbreviation).
'bad' becomes 'ill' (similar in meaning).
'b'+'r'+'ill'='BRILL'
(Other definitions for brill that I've seen before include "catch, perhaps" , "marvellous" , "Excellent (abbr) - fish" , "Very good - fish" , "Fab -- fish" .)