Preparation of tarred fibre used in shipbuilding for caulking (5)

I believe the answer is:

oakum

(Other definitions for oakum that I've seen before include "One hand-picked" , "Fibre from unravelled old rope" , "What convicts used to pick" , "Tarred rope" , "Rope fibre for caulking wooden ships" .)

I've seen this clue in The Independent.

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