A fish out of river at right angles to a vessel (5)
I believe the answer is:
abeam
'at right angles to a vessel' is the definition.
(at right angles on a ship)
'a fish out of river' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
'river' could be 'ea' (I've seen this in other clues) and 'ea' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for abeam that I've seen before include "To the right or left of the centre of the ship" , "In a line at right angles to a vessel's length" , "across centre" , "neither fore nor aft?" , "at right angles to the plane" .)