Fish breathing organ (4)
I believe the answer is:
gill
'fish 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 "Organ for breathing under water" , "Respiratory organ of fish" , "One quarter of a pint" , "Eric ---, sculptor; breathing organ" , "Breathing organ in fish" .)