Two cups affect the quality of porcelain (5)
I believe the answer is:
imari
'porcelain' is the definition.
(I've seen this before)
'two cups affect the quality' is the wordplay.
'two' becomes 'II' (Roman numerals).
'cups' means one lot of letters goes inside another (inserted letters are held or cupped).
'affect the quality' becomes 'mar' (I can't justify this - if you can you should give a lot more credence to this answer).
'ii' enclosing 'mar' is 'IMARI'.
'of' is the link.
Can you help me to learn more?
(Other definitions for imari that I've seen before include "Pottery" , "Japanese earthenware" , "China from Japan" , "Jap. Porcelain" , "Japanese ware" .)