Farmers opt mistakenly for unsuitable agricultural terrain (10)
'unsuitable agricultural terrain' is the definition.
(layer of frozen soil)
'farmers opt mistakenly' is the wordplay.
'mistakenly' indicates anagramming the letters.
'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".)