The most common and most likely answer to this crossword clue is the 2 plus 4 letter word MY FOOT
M
Y
F
O
O
T
More clues leading to the result MY FOOT
- What you find at bottom of the author’s bunk!
- Infantry under 50% of army? Never!
- Rubbish I used to kick?
- I don’t believe you have setter’s measure
- Legend of Tees? It’s rubbish
- No way I’m hopping on that!
- What Oxford may hold for Giovanni? Nonsense!
- –Setter’s stocking filler? I’m not buying that!
- One needing money to pay the bill? No kidding!
- Absolutely not where I put sock!
Buddy explains!

The straight part
We know that "I don’t believe you" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i don’t believe you = my foot.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.