I go off course in SA mountain range (6)
I believe the answer is:
sierra
'mountain range' is the definition.
(a sierra is a chain of mountains)
'i go off course in sa' is the wordplay.
'go off course' becomes 'err' (I've seen this in another clue).
'in' is an insertion indicator.
'i'+'err'='ierr'
'ierr' put into 'sa' is 'SIERRA'.
(Other definitions for sierra that I've seen before include "- - Leone" , "that Romeo is not after" , "Model of car from Ford" , "Mountain range -- code word for S" , "Spanish word for a mountain" .)