The most common and most likely answer to this crossword clue is the 6 letter word OAFISH
I've seen this crossword clue in The Guardian
O
A
F
I
S
H
More clues leading to the result OAFISH
- Uncouth old swimmer crossing centre of bay
- Clumsy poetic angler might announce this
- One of the Ducks on a Pond’s residents, stupid!
- Stupid remark after online discovery?
- Stupid swimmer follows after middle of moat
- Uncouth cry of startled angler!
- Duck, a swimmer lacking elegance?
- Coarse content of shoals, in more ways than one
- Love a swimmer appearing stupid
- Swimmer after coral stripping away odd bits — that’s stupid
Buddy explains!

The straight part
We know that "Stupid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stupid = oafish.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 OAFISH mean?
OAFISH adjective- Characteristic of or resembling an oaf; clumsy, stupid.