Most popular at the box office and thoroughly U (3,6)
I believe the answer is:
top drawer
'u' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for top drawer that I've seen before include "The very best" , "upper class" , "Highest level of society" , "Highest social position - or sliding furniture box" , "Highest social position - a furniture metaphor" .)