Docked in unusually still sea (8)

I believe the answer is:

tailless

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'docked in' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'unusually still sea' is the wordplay.
'unusually' is an anagram indicator.
'still'+'sea'='stillsea'
'stillsea' anagrammed gives 'TAILLESS'.

Can you help me to learn more?

(Other definitions for tailless that I've seen before include "Physical characteristic of Manx cat" , "Like frogs and toads, say" , "Physical condition of Manx cat" .)

I've seen this clue in The Independent.

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