Puzzle concerning transport (5)
I believe the answer is:
rebus
'puzzle' is the definition.
(type of visual puzzle)
'concerning transport' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'transport' becomes 'bus' (busing is a kind of transporting).
're'+'bus'='REBUS'
(Other definitions for rebus that I've seen before include "Visual pun" , "Pun picture puzzle" , "In Edinburgh, busy" , "Picture-letter puzzle" , "Pictorial teaser" .)