It’s bloody to travel by train (4)
I believe the answer is:
gory
'it's bloody' is the definition.
(similar in meaning)
'travel by train' is the wordplay.
'travel' becomes 'go' (synonyms).
'by' says to put letters next to each other.
'train' becomes 'ry' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'go'+'ry'='GORY'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for gory that I've seen before include "Involving much blood" , "Maybe gruesome" , "Involving much bloodshed" , "Steeped in blood" , "Bloodstained" .)