The most common and most likely answer to this crossword clue is the 9 letter word PARACHUTE
I've seen this crossword clue in The Guardian
P
A
R
A
C
H
U
T
E
More clues leading to the result PARACHUTE
- Equipment that can always let you down
- We may hear soldier bring down retardant from plane
- What brings you down is a car up the duff
- People securing building after old man triggers escape mechanism
- As normal, pilot when outside shed safety gear
- Light fixture that helps passengers leaving the plane
- Drop safety equipment
- Drop equipment
- The old man has to run around building that may be coming down
- You’d want me in an emergency? I’d let you down!
Buddy explains!

The straight part
We know that "retardant from plane" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case retardant from plane = parachute.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 PARACHUTE mean?
PARACHUTE noun- A device, generally constructed from fabric, that is designed to employ air resistance to control the fall of an object.
- A web or fold of skin extending between the legs of gliding mammals, such as the flying squirrel and colugo.
- (BDSM) A small collar which fastens around the scrotum and from which weights can be hung.
- To jump, fall, descend, etc. using such a device.
- To introduce into a place using such a device.
- To place (somebody) in an organisation in a position of authority without their having previous experience there; used with in or into.