Dangerous hillside collapsed (4)
I believe the answer is:
fell
This clue is a triple definition!
'dangerous' is the first definition.
(fell can mean fearsome or formidable)
'hillside' is the second definition.
(fell can mean a hill)
'collapsed' is the third definition.
(I've seen this before)
(Other definitions for fell that I've seen before include "Lake District hill" , "Upland tract" , "Chop down" , "Knock down - or was knocked down?" , "Bring down - hill" .)