The most common and most likely answer to this crossword clue is the 5 letter word AHEAD
I've seen this crossword clue in The Guardian
A
H
E
A
D
Buddy explains!

The straight part
We know that "at an advantage" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at an advantage = ahead.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 AHEAD mean?
AHEAD adverb- In or to the front; in advance; onward.
- In the direction one is facing or moving.
- In or for the future.
- At an earlier time.
- Having progressed more.