The most common and most likely answer to this crossword clue is the 6 letter word DOTARD
I've seen this crossword clue in The Guardian
D
O
T
A
R
D
More clues leading to the result DOTARD
- Party getting sluggish, not quite all there?
- Geriatric‘s party I’m a bit annoyed about
- Old fool makes a full stop approaching a road
- Party given by Jack Dee, we hear, for senile fellow
- I’d most hatred, on and off, for old fool
- Cheat thanks Romeo with daughter who’s a feeble-minded person
- Oldie in party running late with yen to leave
- Cheat mostly slow old fool
- Weak-minded person to finish late, falling short
- Senior citizen, possibly fat, shedding pounds after a period
Buddy explains!

The straight part
We know that "old fool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old fool = dotard.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 DOTARD mean?
DOTARD noun- An old person with impaired intellect; one in his or her dotage.
- One who dotes on another, showing excessive fondness.