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

The straight part
We know that "dealer in Cheddar?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dealer in cheddar? = dairyman.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 DAIRYMAN mean?
DAIRYMAN noun- A man who works in a dairy.
- A man who delivers dairy products.