Unorthodox climbers might fall thus (3,3,4)
I believe the answer is:
off the wall
This clue is a double definition.
'unorthodox' is the first definition.
'climbers might fall thus' is the second definition.
(how people on a climbing wall might fall off)
(Other definitions for off the wall that I've seen before include "Peeling wallpaper" , "How Alf left (anag) -- very unconventional" , "Humpty Dumpty's downfall was so" , "American eccentric" , "Implausible" .)