Leaving harbour behind as an after-dinner ritual (7,3,4)

'an after-dinner ritual' is the definition.
I know nothing about this answer so I can't judge whether this works.

'leaving harbour behind as' is the wordplay.
I cannot quite see how this works, but
'leaving' could be 'ort' (I've seen this in another clue) and 'ort' is present in the answer.
'behind' could be 'ass' (both can mean one's back side) and 'ass' is found in the answer.
'as' could be 'in' (I've seen this in other clues) and 'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?