Waders start to slip on rocks (6)
I believe the answer is:
stilts
'waders' is the definition.
(stilt is a kind of wader)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stilts that I've seen before include "Walking poles; birds" , "Sticks to wall on" , "Tall sticks for lofty walking" , "Poles used for walking high above the ground" , "Sticks to help you walk tall" .)