Farmers opt mistakenly for unsuitable agricultural terrain (10)
I believe the answer is:
permafrost
'unsuitable agricultural terrain' is the definition.
(layer of frozen soil)
'farmers opt mistakenly' is the wordplay.
'mistakenly' indicates anagramming the letters.
'farmers'+'opt'='farmersopt'
'farmersopt' with letters rearranged gives 'PERMAFROST'.
'for' is the link.
(Other definitions for permafrost that I've seen before include "Hard layer" , "feature of polar regions" , "Ground frozen for a long time" , "Ground never thawing" .)