Waterproof paints with ink ground (7)
I believe the answer is:
oilskin
'waterproof' is the definition.
(oilskin is a kind of waterproof)
'paints with ink ground' is the wordplay.
'paints' becomes 'oils' (oil is a kind of paint).
'with' says to put letters next to each other.
'ground' is an anagram indicator ('grind' the letters into a new form).
'ink' anagrammed gives 'kin'.
'oils'+'kin'='OILSKIN'
(Other definitions for oilskin that I've seen before include "Waterproof cloth for rain-gear" , "protective cloth" , "security for a rainy day" , "material" , "protection for Ainslie?" .)