Simultaneous arrivals come first in empty trains (5)
I believe the answer is:
twins
'simultaneous arrivals' is the definition.
(children who arrive in the world at the same time)
'come first in empty trains' is the wordplay.
'come first' becomes 'win' (I've seen this before).
'in' indicates putting letters inside.
'empty' means to remove the middle letters.
'trains' with its middle removed is 'ts'.
'win' inserted within 'ts' is 'TWINS'.
(Other definitions for twins that I've seen before include "Gemini" , "Two offspring born at the same time to the same mother" , "Two born together" , "Matching pairs" , "Sign" .)