Crossword Genius

Farmers opt mistakenly for unsuitable agricultural terrain (10)

Ross

I believe the answer is:

'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".)

I've seen this clue in The Times.