Small blue vehicle, one lagging behind (9)
I believe the answer is:
'one lagging behind' is the definition.
'small blue vehicle' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'blue' becomes 'low' (I've seen this in other clues).
'vehicle' becomes 'coach' (coach is a kind of vehicle**).
's'+'low'+'coach'='SLOWCOACH'
(Other definitions for slowcoach that I've seen before include "One who is sluggish in work or actions, due to tardy vehicle?" , "Person (or transport?) taking a long time" , "Snail" , "plodder" , "Tortoise-like walker" .)