Mountain trail twisting round historic Indian province (6)
I believe the answer is:
bengal
'historic indian province' is the definition.
(Bengal was a province of British India)
'mountain trail twisting round' is the wordplay.
'mountain' becomes 'ben' (ben is a kind of mountain).
'trail' becomes 'lag' (both can mean to go behind).
'twisting round' is a reversal indicator.
'lag' reversed gives 'gal'.
'ben'+'gal'='BENGAL'
(Other definitions for bengal that I've seen before include "old Indian province" , "Bay of --; -- tiger" , "found in part of subcontinent" , "Former province now divided between India and Bangladesh" , "Bay of - is E. of India" .)