Fish a stream at Birkenhead (5)
I believe the answer is:
brill
'fish' is the definition.
(brill is a kind of fish)
'a stream at birkenhead' is the wordplay.
'a stream' becomes 'rill' (rill is a kind of stream).
'at' means one lot of letters go next to another (I've seen this in other clues).
'birkenhead' becomes 'b' (head letter of 'birken').
'rill' put after 'b' is 'BRILL'.
(Other definitions for brill that I've seen before include "catch, perhaps" , "swimmer" , "Great" , "marvellous" , "Very good - fish" .)