Puzzle concerning passenger transport (5)
I believe the answer is:
rebus
'puzzle' is the definition.
(type of visual puzzle)
'concerning passenger transport' is the wordplay.
I cannot really see how this works, but
'concerning' could be 're' (both can mean on a particular topic) and 're' is located in the answer.
'transport' could be 'bus' (busing is a kind of transporting) and 'bus' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rebus that I've seen before include "Picture and word puzzle" , "Word-picture puzzle" , "Puzzling device" , "Problem" , "Word puzzle with pictures and letters" .)