Slip over area in metal country (7)
I believe the answer is:
terrain
'country' is the definition.
(terrain means some land or country)
'slip over area in metal' is the wordplay.
'slip' becomes 'err' (to err is to make an error).
'over area' becomes 'a' (maths abbreviation. I am not sure about the 'over' bit.).
'in' is an insertion indicator.
'metal' becomes 'tin' (tin is a kind of metal).
'err'+'a'='erra'
'erra' placed into 'tin' is 'TERRAIN'.
Can you help me to learn more?
(Other definitions for terrain that I've seen before include "Stretch of land, emphasising natural features" , "Land (viewed tactically)" , "Land of a particular type" , "Tract of land with prevailing characteristic" , "Land with respect to its features" .)