Fur coat and no knickers, primarily? Lass that is false (8)
I believe the answer is:
sealskin
'fur coat and no knickers primarily? lass that is false' is the definition.
'sealskin' can be an answer for 'fur coat'. I am not certain of the rest of the definition.
'fur coat' is the wordplay.
'fur' becomes 'seal' (seal is a kind of fur).
'coat' becomes 'skin' (I've seen this before).
'seal'+'skin'='SEALSKIN'
(Other definitions for sealskin that I've seen before include "prepared fur" , "Form of fur for garments" , "Fur from the sea" , "Fur - lean kiss (anag)" , "Marine fur" .)