In fiction, an inspector on public transport (5)
I believe the answer is:
rebus
'in fiction an inspector' is the definition.
(Inspector John Rebus in books by Ian Rankin)
'on public transport' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'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" , "Pictorial teaser" , "Puzzling symbols, etc" , "Puzzling device" , "Picture-letter puzzle" .)