I go astray in South American mountain range (6)
I believe the answer is:
sierra
'mountain range' is the definition.
(a sierra is a chain of mountains)
'i go astray in south american' is the wordplay.
'go astray' becomes 'err' (to err is to make an error or go the wrong way).
'in' means one lot of letters goes inside another.
'south american' becomes 'SA'.
'i'+'err'='ierr'
'ierr' going inside 'sa' is 'SIERRA'.
(Other definitions for sierra that I've seen before include "Spanish word for a mountain" , "Spanish range of mountains" , "- - Leone" , "Mountain range -- code word for S" , "that Romeo is not after" .)