Prolific correspondent impresses sailor in waterproof material (9)
I believe the answer is:
tarpaulin
This clue could be a double definition.
'prolific correspondent impresses sailor' is the first definition.
'tarpaulin' can be an answer for 'sailor' (thesaurus). I am unsure of the 'prolific correspondent impresses' bit.
'waterproof material' is the second definition.
(I've seen this in another clue)
'in' is the link.
Can you help me to learn more?
(Other definitions for tarpaulin that I've seen before include "protective cover" , "canvas given treatment?" , "Heavy waterproof canvas" , "seaman rarely required" , "It provides protection" .)