Fish bleeding briefly (4)
I believe the answer is:
rudd
'fish' is the definition.
(rudd is a kind of fish)
I cannot understand how the remainder of the clue works.
(Other definitions for rudd that I've seen before include "Freshwater fish (silvery with red fins)" , "Freshwater fish of the carp family" , "Swimmer" , "Red-eye fish" , "European freshwater fish" .)