Dull whisky (litre declined) (3)
I believe the answer is:
mat
'dull' is the definition.
(mat as opposed to glossy)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for mat that I've seen before include "Thick flat pad on the floor" , "Opening of good book" , "coaster" , "Shoe-cleaner" , "Small rug" .)