Rankin's inspector on public transport (5)
I believe the answer is:
rebus
'rankin's inspector' is the definition.
(I've seen this before)
'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 "Visual type of word puzzle" , "Puzzling device" , "Puzzling symbols, etc" , "Picture and word puzzle" , "In Edinburgh, busy" .)