Second-class in tube train leaving Turin (4)
I believe the answer is:
beta
'second-class' is the definition.
(one less than alpha)
'tube train leaving turin' is the wordplay.
'leaving' suggests deleting specific letters.
'tube'+'train'='tubetrain'
'tubetrain' with 'turin' removed is 'BETA'.
'in' is the link.
(Other definitions for beta that I've seen before include "second class grade?" , "less than perfect mark" , "Old character" , "Second Greek letter - beat that" , "Greek letter -- second in order of importance" .)