One soon up in Derby with rail alteration (5,4)
I believe the answer is:
early bird
'rail alteration' is the definition.
The definition suggests a singular noun which matches the answer.
'one soon up in derby' is the wordplay.
'one soon' becomes 'early' (I've seen this before).
'up in derby' becomes 'bird' (I can't explain this - if you can you should give a lot more credence to this answer).
'early'+'bird'='EARLY BIRD'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for early bird that I've seen before include "Worm catcher?" , "arrived before time" , "first to start in the morning" , "This one catches the worm" , "One up with the lark" .)