The most common and most likely answer to this crossword clue is the 7 letter word CAVIARE
I've seen this crossword clue in The Guardian
C
A
V
I
A
R
E
Buddy explains!

The straight part
We know that "a staple for the rich" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a staple for the rich = caviare.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 CAVIARE mean?
CAVIARE noun- Roe of the sturgeon or other large fish, considered a delicacy.
- Something whose flavour is too fine for the vulgar taste.