A lungful of sea air? (4,6)

I believe the answer is:

deep breath

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'a lungful' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'of sea air?' is the wordplay.
'of sea' becomes 'deep' (I am not sure about this - if you are sure you should believe this answer much more).
'air?' becomes 'breath' (breath is a kind of air).
'deep'+'breath'='DEEP BREATH'

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I've seen this clue in the Evening Standard.

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