Opens with tuna, essentially, and other marine creatures (7)

I believe the answer is:

unseals

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'opens' is the definition.
(unsealing is a kind of opening)

'tuna essentially and other marine creatures' is the wordplay.
'essentially' indicates the central letters.
'and' says to put letters next to each other.
'other marine creatures' becomes 'seals' (I can't justify this - if you can you should believe this answer much more).
The centre of 'tuna' is 'un'.
'un'+'seals'='UNSEALS'

'with' acts as a link.

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I've seen this clue in The Irish Times.

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