Small ridge of rock does for toboggan (6)
I believe the answer is:
sledge
'for toboggan' is the definition.
(I've seen this before)
'small ridge' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ridge' becomes 'ledge' (ledge is a kind of ridge).
's'+'ledge'='SLEDGE'
'of rock does' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sledge that I've seen before include "Heavy long-handled hammer" , "Put cricketer off" , "Vehicle for travelling over snow" , "heavy tool" , "In Canada train" .)