The most common and most likely answer to this crossword clue is the 4 letter word AERO
I've seen this crossword clue in Independent
A
E
R
O
More clues leading to the result AERO
Buddy explains!

The straight part
We know that "a bar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a bar = aero.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 AERO mean?
AERO noun- Aerodynamics.
- An airplane or airship.
- Aerospace.
- Of or pertaining to aviation.
- Aerodynamic; having an aerodynamic appearance.