Control a toboggan rising in speed (8)
I believe the answer is:
regulate
'control' is the definition.
(I've seen this before)
'a toboggan rising in speed' is the wordplay.
'a toboggan' becomes 'luge' (I've seen this before).
'rising' shows that the letters should be reversed in order (going up in a down clue).
'in' is an insertion indicator.
'speed' becomes 'rate' (the speed at which something happens).
'luge' backwards is 'egul'.
'egul' inserted inside 'rate' is 'REGULATE'.
(Other definitions for regulate that I've seen before include "Bring order" , "ADjust, fix" , "Control, supervise" , "Police" , "Adjust - control" .)