The most common and most likely answer to this crossword clue is the 11 letter word INTERCEPTOR
I've seen this crossword clue in The Guardian
I
N
T
E
R
C
E
P
T
O
R
Buddy explains!

The straight part
We know that "Fighter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fighter = interceptor.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 INTERCEPTOR mean?
INTERCEPTOR noun- Anything that intercepts something else.
- A fast, maneuverable fighter aircraft designed to intercept and destroy enemy aircraft before they can attack.
- A guided missile designed to intercept and destroy enemy missiles.
- A device to trap, remove, or separate deleterious, hazardous, or undesirable matter (such as oil, grease, gasoline, sand, and sediment) from normal waste conveyed through it, permitting normal sewage or liquid wastes to discharge into the disposal terminal by gravity.
- A hook routine that intercepts normal program flow to carry out a task.
- A sewer which intercepts sewerage destined for an outfall (e.g. into a river) and redirects it to sewerage treatment plant