Coastal features - aquatic mammals with a large inside (3,5)

I believe the answer is:

sea walls

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'coastal features aquatic' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'mammals with a large inside' is the wordplay.
'mammals' becomes 'seals' (seal is a kind of mammal).
'with' becomes 'w' (abbreviation).
'large' becomes 'L' (eg in clothes sizes).
'inside' is an insertion indicator.
'w'+'a'+'l'='wal'
'seals' going around 'wal' is 'SEA-WALLS'.

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I've seen this clue in The Independent.

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