Ridge of free design (4)
I believe the answer is:
reef
'ridge' is the definition.
(I know that reef is a type of ridge)
'free design' is the wordplay.
'design' indicates anagramming the letters.
'free' with letters rearranged gives 'REEF'.
'of' is the link.
(Other definitions for reef that I've seen before include "Marine formation" , "Submerged ridge near the surface of the sea" , "The Great Barrier ..., Australian natural wonder" , "Hazard for ships" , "Sandbank" .)