Crossword Genius

A lighthouse casts it across the vessel (5)

Ross

I believe the answer is:

abeam

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.
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