Sort of ground farmers opt to change (10)
I believe the answer is:
permafrost
'sort of ground' is the definition.
'farmers opt to change' is the wordplay.
'to change' indicates an anagram.
'farmers'+'opt'='farmersopt'
'farmersopt' with letters rearranged gives 'PERMAFROST'.
(Other definitions for permafrost that I've seen before include "Ground frozen for a long time" , "Hard layer" , "Ground never thawing" , "feature of polar regions" .)