Control toboggan rising in speed (8)
I believe the answer is:
regulate
'control' is the definition.
(thesaurus)
'toboggan rising in speed' is the wordplay.
'toboggan' becomes 'luge' ('luge' can be a synonym of 'toboggan').
'rising' says the letters should be written backwards (going up in a down clue).
'in' indicates putting letters inside.
'speed' becomes 'rate' (rate is a kind of speed**).
'luge' reversed gives 'egul'.
'egul' going within 'rate' is 'REGULATE'.
(Other definitions for regulate that I've seen before include "Control or direct by means of rules and procedures" , "Police" , "Ate gruel (anag.)" , "Bring order" , "Adjust - control" .)