Crossword Genius

On what notices are often on (6)

Ross

I believe the answer is:

aboard

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

'on' is the definition.
Both the definition and answer are adverbs. Maybe there's a link between them I don't understand?

'notices are often on' is the wordplay.
I cannot really understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
The remaining letters 'board' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'what' is the link.

Can you help me to learn more?

(Other definitions for aboard that I've seen before include "Joined for mutual benefit" , "on a yacht maybe" , "on deck" , "A poet penning nothing on vessel" , "on a ferry?" .)

I've seen this clue in the Evening Standard.
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