Subject takes railway line (6)
I believe the answer is:
tropic
'line' is the definition.
(I know that tropic is a type of latitude)
'subject takes railway' is the wordplay.
I cannot really understand how this works, but
'railway' could be 'r' (eg in Great Western Railway) and 'r' is located in the answer.
'subject' could be 'topic' (synonyms) and 'topic' is found within the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tropic that I've seen before include "Latitude twenty-three degs twenty-six mins north or south" , "Latitude 23 and a quarter 26 N or S" , "One of two parallels of latitude" , "Cancer/Capricorn line" , "Line of latitude determined by the solstice" .)