The problem about public transport (5)
I believe the answer is:
rebus
'the problem' is the definition.
(type of puzzle or problem)
'about public transport' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'public transport' becomes 'bus' (bus is a kind of public transport).
're'+'bus'='REBUS'
(Other definitions for rebus that I've seen before include "Puzzle where pictures represent syllables and words" , "Puzzling symbols, etc" , "Visual wordplay - Rankin's inspector" , "Puzzling device" , "Pictorial pun - fictional detective" .)