Most of Asian country penetrated by a train (5)
I believe the answer is:
chain
'train' is the definition.
(thesaurus)
'most of asian country penetrated by a' is the wordplay.
'most of' means to remove the last letter.
'asian country' becomes 'China' (country in Asia).
'penetrated by' indicates putting letters inside (I've seen this in other clues).
'china' with its final letter removed is 'chin'.
'chin' enclosing 'a' is 'CHAIN'.
(Other definitions for chain that I've seen before include "Range of mountains" , "Succession" , "Group of atoms" , "trading group" , "unwelcome to house seeker" .)