Shelf on which sits small toboggan (6)
I believe the answer is:
sledge
'toboggan' is the definition.
(I've seen this before)
'shelf on which sits small' is the wordplay.
I cannot really see how this works, but
'shelf' could be 'ledge' (synonyms) and 'ledge' is present in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sledge that I've seen before include "Vehicle on runners, for snow and ice" , "Toboggan" , "one could go downhill fast" , "Vehicle for transportation over snow" , "Make offensive remarks" .)