Ill humour of top signalmen and shunters is about old city lines (9)
I believe the answer is:
surliness
'ill humour of top signalmen and shunters' is the definition.
I can't tell whether this defines the answer.
'about old city lines' is the wordplay.
'about' means one lot of letters goes inside another.
'old' becomes 'ss' (I am not sure about this - if you are sure you should believe this answer much more).
'city' becomes 'Ur' (ancient Sumerian city state).
'ur'+'lines'='urlines'
'ss' going around 'urlines' is 'SURLINESS'.
'is' is the link.
Can you help me to learn more?
(Other definitions for surliness that I've seen before include "unfriendly state" , "Sullen churlishness" , "Biliousness, irritability" .)