Toboggan beginning to slide over projecting shelf (6)
I believe the answer is:
sledge
'toboggan' is the definition.
(I've seen this before)
'beginning to slide over projecting shelf' is the wordplay.
'beginning to' indicates taking the first letters.
'over' means one lot of letters go next to another.
'projecting shelf' becomes 'ledge' (I've seen this before).
The initial letter of 'slide' is 's'.
's'+'ledge'='SLEDGE'
(Other definitions for sledge that I've seen before include "In Canada train" , "Snow conveyance" , "Heavy long-handled hammer" , "Vehicle on runners, for snow and ice" , "Make offensive remarks" .)