Fish bypassing top stream (5)
I believe the answer is:
brill
'fish' is the definition.
(brill is a kind of fish)
'bypassing top stream' is the wordplay.
'top' suggests taking the first letters (in a down clue, the first letter is top).
'stream' becomes 'rill' (rill is a kind of stream).
The initial letter of 'bypassing' is 'b'.
'b'+'rill'='BRILL'
(Other definitions for brill that I've seen before include "swimmer" , "Flatfish; excellent" , "Excellent (abbr) - fish" , "First-class (informal) - fish" , "Very good - fish" .)