The most common and most likely answer to this crossword clue is the 6 letter word DUFFER
I've seen this crossword clue in The Daily Telegraph
D
U
F
F
E
R
Buddy explains!
The straight part
We know that "Stupid person" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stupid person = duffer.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 DUFFER mean?
DUFFER adjective- Worthless; not working properly, defective.
- An incompetent or clumsy person.
- A player having little skill, especially a golfer who duffs.
- A pedlar or hawker, especially one selling cheap or substandard goods.
- Cheap or substandard goods sold by a duffer.
- A cow that does not produce milk.
- A cattle thief or thief of other livestock; one who alters the brands of cattle.
- A racing pigeon that does not perform well.
- Any of various nymphalid butterflies of the genus Discophora.