The most common and most likely answer to this crossword clue is the 7 plus 7 plus 5 letter word ULYSSES SIMPSON GRANT
I've seen this crossword clue in The Guardian
U
L
Y
S
S
E
S
S
I
M
P
S
O
N
G
R
A
N
T
Buddy explains!
The straight part
We know that "President" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case president = ulysses simpson grant.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.