Crossword Genius

On terra firma, one can still be all at sea! (6)

Ross

I believe the answer is:

ashore

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

'on terra firma' is the definition.
(I've seen this in another clue)

'one can still be all at sea' is the wordplay.
I cannot really see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'shore' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for ashore that I've seen before include "On land (as opposed to the sea)" , "Hoarse being no longer at sea" , "Hoarse (anag)" , "Toward land from water" , "in port?" .)

I've seen this clue in the Evening Standard.
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