Central Yemen railway is smoother (5)
I believe the answer is:
emery
'is smoother' is the definition.
(emery paper is used to smooth things)
'central yemen railway' is the wordplay.
'central' means to look at the middle letters.
'railway' becomes 'ry' (abbreviation for railway).
The middle letters of 'yemen' are 'eme'.
'eme'+'ry'='EMERY'
(Other definitions for emery that I've seen before include "Rock used as abrasive" , "Hard grey-black mineral used as an abrasive" , "Abrasive powder" , "Dick - - , late comedian" , "old comedian" .)