Crossword Genius

Informal name that means nothing to a sailor? (5)

Ross

I believe the answer is:

jacko

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

'informal name that means nothing to a' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'sailor?' is the wordplay.
I cannot really understand how this works, but
'sailor?' could be 'jack' (I've seen this before) and 'jack' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for jacko that I've seen is " - - - -lantern".)

I've seen this clue in the Evening Standard.
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