Running across a fish leaving river (5)
I believe the answer is:
abeam
'running across' is the definition.
(nautical term)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for abeam that I've seen before include "At right angles to ship's length" , "at right angles to a ship's keel" , "across the deck" , "The way sailor goes across ship" , "at right angles to vessel" .)