Moorland collapsed (4)
I believe the answer is:
fell
I believe this is a double definition.
'moorland' is the first definition.
(fell can mean a moor)
'collapsed' is the second definition.
(I've seen this before)
(Other definitions for fell that I've seen before include "Brutal -- high moorland" , "Upland tract" , "Hew; tumbled" , "yielded to attack" , "Hide" .)