The most common and most likely answer to this crossword clue is the 6 plus 5 letter word PUERTO RICAN
I've seen this crossword clue in The Times
P
U
E
R
T
O
R
I
C
A
N
Buddy explains!
The straight part
We know that "from San Juan?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case from san juan? = puerto rican.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.