Reptile mostly black going to the West Country (6)
I believe the answer is:
brazil
'country' is the definition.
(I know that Brazil is a South American country)
'reptile mostly black going to the west' is the wordplay.
'reptile' becomes 'lizard' (lizard is a kind of reptile).
'mostly' means to remove the last letter (most but not all of the word).
'black' becomes 'b' (abbreviation as in pencil hardness).
'going to the west' is a reversal indicator.
'lizard' with its last letter removed is 'lizar'.
'lizar'+'b'='lizarb'
'lizarb' back-to-front is 'BRAZIL'.
(Other definitions for brazil that I've seen before include "Country Charley's Aunt came from" , "South American state" , "Nut (from South America?)" , "Football World Cup holders" , "Country of Rio de Janeiro and Sao Paulo" .)