Quick way to reach rock bottom? (6)
I believe the answer is:
abseil
'quick way to reach rock bottom?' is the definition.
(means of getting to the bottom of a rock)
This is the entire clue.
(Other definitions for abseil that I've seen before include "Descend rock-face using a doubled rope - like Isabel" , "Descend down vertical surface using rope" , "Go down by rope" , "Descend by rope from helicopter" , "Descend vertical slope using rope" .)