Picture puzzle about public transport (5)
I believe the answer is:
rebus
'picture puzzle' is the definition.
(I've seen this in another clue)
'about public transport' is the wordplay.
'about' becomes 're' (regarding).
'public transport' becomes 'bus' (bus is a kind of public transport).
're'+'bus'='REBUS'
(Other definitions for rebus that I've seen before include "Enigmatic representation" , "Picture and word puzzle" , "that's puzzling" , "Visual type of word puzzle" , "Pictorial pun - fictional detective" .)