One who lags behind bus that stops everywhere?
I believe the answer is:
slowcoach
'one who lags behind bus that stops everywhere?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one who lags behind bus' is the wordplay.
'one who lags' becomes 'slow' (I can't explain this - if you can you should give a lot more credence to this answer).
'behind' says to put letters next to each other.
'bus' becomes 'coach' (synonyms).
'slow'+'coach'='SLOWCOACH'
Can you help me to learn more?
(Other definitions for slowcoach that I've seen before include "Not a fast worker" , "One who is sluggish in work or actions, due to tardy vehicle?" , "Tortoise-like walker" , "certainly not one of the fleet" , "One who takes time to do things" .)