Completely out of port and unable to explain why? (3,2,3)

I believe the answer is:

all at sea

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'unable to explain why?' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'completely out of port' is the wordplay.
'completely' becomes 'all' (synonyms).
'out of port' becomes 'atsea' (I've seen this in another clue).
'all'+'atsea'='ALL AT SEA'

'and' acts as a link.

Can you help me to learn more?

(Other definitions for all at sea that I've seen before include "state of confusion" , "Nobody in the dock" , "Entire fleet's" , "Entire crew" , "Completely confused on voyage?" .)

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

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