Nude form of hill (4)
I believe the answer is:
dune
'hill' is the definition.
(I know that dune is a type of hill)
'nude form' is the wordplay.
'form' indicates an anagram (another form of the letters).
'nude' with letters rearranged gives 'DUNE'.
'of' is the link.
(Other definitions for dune that I've seen before include "Nude in the sand" , "Sandy hill" , "Ridge of sand created by wind" , "Film" , "Ridge of wind-blown sand" .)