Dull dare, running by railway (6)
I believe the answer is:
dreary
'dull' is the definition.
(similar in meaning)
'dare running by railway' is the wordplay.
'running' indicates anagramming the letters (the letters run around to different places).
'by' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation).
'dare' anagrammed gives 'drea'.
'drea'+'ry'='DREARY'
(Other definitions for dreary that I've seen before include "Overcast" , "Dismal, depressing" , "Drab and dismal" , "Dismal, funeral" , "Bleak, depressing" .)