A lighthouse casts it across the vessel (5)
I believe the answer is:
abeam
'casts it across the vessel' is the definition.
I can't tell whether this defines the answer.
'a lighthouse' is the wordplay.
'lighthouse' becomes 'beam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'beam'='ABEAM'
Can you help me to learn more?
(Other definitions for abeam that I've seen before include "at right angles to vessel" , "neither fore nor aft?" , "at right angles to a ship's keel" , "from side to side on voyage?" , "at right angles to the plane" .)