Dalmatians unlikely to be this pure (8)
I believe the answer is:
spotless
'this pure' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for spotless that I've seen before include "Spic-and-span" , "Without a mark, completely clean" , "Extremely clean" , "Fully clean" , "Immaculate" .)