You'd want me in an emergency? I'd let you down! (9)
I believe the answer is:
parachute
'down' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for parachute that I've seen before include "Equipment for making a descent from an airplane" , "Eat up char for something to break your fall" , "Drop from plane" , "It breaks a fall from the sky" , "canopy" .)