Two setters in water — this keeps us dry (9)
I believe the answer is:
tarpaulin
'this keeps us dry' is the definition.
(tarpaulin is a waterproof material)
'two setters in water' is the wordplay.
'two setters' means two replacements for 'setter'.
'setter' becomes 'paul' (Paul is a Guardian crossword setter).
'setter' becomes 'I' (the person that set the crossword).
'in' indicates putting letters inside.
'water' becomes 'tarn' (tarn is a type of lake).
'paul'+'i'='pauli'
'pauli' placed within 'tarn' is 'TARPAULIN'.
(Other definitions for tarpaulin that I've seen before include "It provides protection" , "sailor's hat" , "waterproof material" , "oilskin hat" , "Waterproofed canvas" .)