Long jagged mountain chain (6)
I believe the answer is:
sierra
'long jagged mountain chain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sierra that I've seen before include "Mountain range in Spanish-speaking countries" , "Mountain chain, S (radio comms.)" , "International codeword" , "------ Nevada, mountains of Spain and California" , "Long jagged mountain chain" .)