Mountain chain called Eastern (5)
I believe the answer is:
range
'mountain chain' is the definition.
('range' can be a synonym of 'mountain chain')
'called eastern' is the wordplay.
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'eastern' becomes 'e'.
'rang'+'e'='RANGE'
(Other definitions for range that I've seen before include "Set in a row" , "Selection" , "Walk for miles" , "Roam" , "Set < compass" .)