The most common and most likely answer to this crossword clue is the 5 letter word ADAMS
I've seen this crossword clue in Financial Times
A
D
A
M
S
More clues leading to the result ADAMS
- Leader of assembly blocks US president
- A tree drops on author
- Notice a lady is president twice
- A mother’s one who wrote animal tales
- First lady’s partner, second president
- US president into communism adapted after a U-turn
- Hydroelectric projects led by American President
- Barrier breaks as US president
- First man and second president
- US president up in arms, mad apparently!
Buddy explains!
The straight part
We know that "old American leader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old american leader = adams.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.