Waterproof paint layer (7)
I believe the answer is:
oilskin
'waterproof' is the definition.
(I know that oilskin is a type of Mackintosh)
'paint layer' is the wordplay.
'paint' becomes 'oil' (oil is a kind of paint).
'layer' becomes 'skin' (I have seen 'Outer layer ' mean 'skin' so perhaps 'layer' could also mean 'skin').
'oil'+'skin'='OILSKIN'
(Other definitions for oilskin that I've seen before include "protective cover" , "protection for Ainslie?" , "Protective coat" , "security for a rainy day" , "Waterproof cloth as for sailors" .)