A commercial vehicle transporting a large red slide (9)
I believe the answer is:
avalanche
'slide' is the definition.
(I know that avalanche is a type of slide)
'a commercial vehicle transporting a large red' is the wordplay.
'commercial vehicle' becomes 'van' (I've seen this before).
'transporting' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'red' becomes 'che' (Che Guevara the Marxist revolutionary).
'a'+'l'='al'
'van' enclosing 'al' is 'valan'.
'a'+'valan'+'che'='AVALANCHE'
(Other definitions for avalanche that I've seen before include "Heavy fall of rock or snow from mountain" , "heavy snowfall" , "flood" , "This dangerous flow from some peaks" , "Slide of a large amount of snow down a mountain" .)