Isle of Sheppey town, site of a former naval dockyard (9)

I believe the answer is:

sheerness

(Other definitions for sheerness that I've seen before include "port" , "Verticality (of, eg, a cliff); town on Isle of Sheppey" , "Perpendicularity; Isle of Sheppey town" , "Kent resort" , "Site of former naval dockyard in Isle of Sheppey" .)

I've seen this clue in The Sunday Times and The Times.

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