Cultivating osier - no natural process (7)
I believe the answer is:
erosion
'natural process' is the definition.
(erosion is a kind of natural process)
'cultivating osier no' is the wordplay.
'cultivating' indicates anagramming the letters.
'osier'+'no'='osierno'
'osierno' anagrammed gives 'EROSION'.
(Other definitions for erosion that I've seen before include "being eaten away" , "Deterioration of soil etc" , "Wearing away (of soil, rocks etc)" , "The wearing away by water or weather" , "Process of wear" .)