One filleting fish perhaps into a trough (6)
I believe the answer is:
gutter
'trough' is the definition.
(I know that trough can be written as gutter)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gutter that I've seen before include "Channel for carrying off water" , "Someone removing contents" , "Person who empties" , "piping?" , "Drainage trough" .)