Cream in milk mopped up by sponge (3,6)
I believe the answer is:
top drawer
'cream' is the definition.
(both can mean the best)
'milk mopped up by sponge' is the wordplay.
'milk' becomes 'draw milk' (synonyms).
'mopped up by' means one lot of letters goes inside another (inserted letters get eaten or 'mopped up').
'sponge' becomes 'toper' (both can mean a drunk).
'draw' going into 'toper' is 'TOP-DRAWER'.
'in' acts as a link.
(Other definitions for top drawer that I've seen before include "Highest social position - or sliding furniture box" , "The very best" , "main attraction" , "Those in high society" , "Of the highest class" .)