The most common and most likely answer to this crossword clue is the 8 letter word CREDENZA
I've seen this crossword clue in The Guardian
C
R
E
D
E
N
Z
A
Buddy explains!

The straight part
We know that "a cupboard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a cupboard = credenza.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CREDENZA mean?
CREDENZA noun- A sideboard or buffet.
- A horizontal filing cabinet, typically placed behind a desk.