Fixed terminus on a railway (10)
I believe the answer is:
stationary
'fixed' is the definition.
(synonyms)
'terminus on a railway' is the wordplay.
'terminus' becomes 'station' (train terminus).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'railway' becomes 'ry' (abbreviation for railway).
'station'+'a'+'ry'='STATIONARY'
(Other definitions for stationary that I've seen before include "Motionless" , "Unmoving" , "Parking" , "Still" , "Not moving" .)