Completely out of port and can't explain why? (3,2,3)

I believe the answer is:

all at sea

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'can't explain why?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'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' is the link.

Can you help me to learn more?

(Other definitions for all at sea that I've seen before include "Entire crew" , "Confused, uncertain" , "Rescuers at Dunkirk were thus" , "Utterly confused, totally on the ocean" , "Completely confused on voyage?" .)

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

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