Mountain range I slip up in South Africa (6)
I believe the answer is:
sierra
'mountain range' is the definition.
(a sierra is a chain of mountains)
'i slip up in south africa' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'in' is an insertion indicator.
'south africa' becomes 'SA' (abbreviation).
'i'+'err'='ierr'
'ierr' inserted inside 'sa' is 'SIERRA'.
(Other definitions for sierra that I've seen before include "International codeword" , "Andalusian mountain range" , "One range" , "Mountain range in Spanish-speaking countries" , "A Spanish chain of mountains" .)