The most common and most likely answer to this crossword clue is the 13 letter word BOBBY-DAZZLER
I've seen this crossword clue in Financial Times
B
O
B
B
Y
-
D
A
Z
Z
L
E
R
Buddy explains!
The straight part
We know that "A gem" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a gem = bobby-dazzler.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 BOBBY-DAZZLER mean?
BOBBY-DAZZLER noun- (chiefly British and Australian) An attractive person.