Jack, boy in waterproof stuff (9)

I believe the answer is:

tarpaulin

'waterproof stuff' is the definition.
(waterproof material)

'jack boy in' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'boy' becomes 'Paul' (boy's name).
'tar'+'paul'+'in'='TARPAULIN'

(Other definitions for tarpaulin that I've seen before include "Waterproofed canvas" , "waterproof material" , "sailor's hat" , "oilskin hat" , "protective cover" .)

I've seen this clue in The Telegraph.

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