Mistake linked to area in output of Cornish mining country (7)
I believe the answer is:
terrain
'country' is the definition.
(terrain means some land or country)
'mistake linked to area in output' is the wordplay.
'mistake' becomes 'err' (to err is to make a mistake or blunder).
'linked to' means one lot of letters go next to another.
'area' becomes 'a' (maths abbreviation).
'in' means one lot of letters goes inside another.
'output' becomes 'tin' (I can't justify this - if you can you should give a lot more credence to this answer).
'err'+'a'='erra'
'erra' inserted within 'tin' is 'TERRAIN'.
'of cornish mining' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for terrain that I've seen before include "Physical features of a tract of land" , "Ground (conditions)" , "Tract of land with particular character" , "Land with respect to its features" , "Stretch of land, emphasising natural features" .)