Fish's breathing organ (4)
I believe the answer is:
gill
'fish's breathing organ' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gill that I've seen before include "Eric ---, sculptor; breathing organ" , "Six nips" , "Respiratory organ of fish" , "Fish's breathing apparatus" , "Organ for breathing under water" .)