Unfortunately have canal fall (9)
I believe the answer is:
avalanche
'fall' is the definition.
(I know that avalanche is a more specific form of the action fall)
'unfortunately have canal' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'have'+'canal'='havecanal'
'havecanal' with letters rearranged gives 'AVALANCHE'.
(Other definitions for avalanche that I've seen before include "This dangerous flow from some peaks" , "Large mass of sliding snow or stone" , "Great snowfall" , "heavy snowfall" , "Large fall of snow or rocks" .)