A chapter head including name of a major mountain system (6)
I believe the answer is:
andean
'of a major mountain system' is the definition.
(I know that Andean is the attribute pertaining to Andes)
'a chapter head including name' is the wordplay.
'chapter head' becomes 'dean' (I've seen this before).
'including' means one lot of letters goes inside another.
'name' becomes 'n' (common abbreviation e.g. on forms).
'a'+'dean'='adean'
'adean' going around 'n' is 'ANDEAN'.
(Other definitions for andean that I've seen before include "description of range?" , "Like Incas or llamas" , "one highly placed in South America" , "within range?" , "Peak District" .)