A light abreast of the vessel (5)
I believe the answer is:
abeam
'abreast of the vessel' is the definition.
I can't judge whether this definition defines the answer.
'a light' is the wordplay.
'light' becomes 'beam' (beam is a kind of light).
'a'+'beam'='ABEAM'
Can you help me to learn more?
(Other definitions for abeam that I've seen before include "across the deck" , "at right angles to vessel" , "To the right or left of the centre of the ship" , "In a line at right angles to a vessel's length" , "going from one side to the other" .)