Small blue train, one lagging behind (9)
I believe the answer is:
slowcoach
'one lagging behind' is the definition.
(slowcoach means a slow person)
'small blue train' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'blue' becomes 'low' (I've seen this in another clue).
'train' becomes 'coach' (synonyms).
's'+'low'+'coach'='SLOWCOACH'
(Other definitions for slowcoach that I've seen before include "trailer" , "Laggard" , "certainly not one of the fleet" , "Snail" , "Not a fast worker" .)