Moorland is deadly (4)
I believe the answer is:
fell
I believe this clue is a double definition.
'moorland' is the first definition.
(I've seen this before)
'is deadly' is the second definition.
(I've seen this before)
(Other definitions for fell that I've seen before include "Bring down - hill" , "'I do not like thee Doctor ....'" , "Stuck-up" , "Knock down - or was knocked down?" , "Cut down; hill" .)