Invaders are beaten on a mountain range (6,6)
I believe the answer is:
sierra nevada
'mountain range' is the definition.
(Sierra Nevada is an example)
'invaders are beaten on a' is the wordplay.
'beaten' is an anagram indicator (letters beaten up or destroyed).
'on' indicates putting letters inside (as in clothing 'on' a person).
'invaders'+'are'='invadersare'
'invadersare' with letters rearranged gives 'sierranevda'.
'sierranevda' enclosing 'a' is 'SIERRA NEVADA'.
(Other definitions for sierra nevada that I've seen before include "Andalusian mountain range" , "Mountains" , "Mountain range in Spain or E. California" , "Range of Western US" .)