The most common and most likely answer to this crossword clue is the 10 letter word QUEUE-JUMP
I've seen this crossword clue in The Times
Q
U
E
U
E
-
J
U
M
P
Buddy explains!
The straight part
We know that "gain an unfair advantage" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gain an unfair advantage = queue-jump.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 QUEUE-JUMP mean?
QUEUE-JUMP verb- Jump the queue