Poor old bums in public vehicle (7)
I believe the answer is:
omnibus
'public vehicle' is the definition.
(I've seen this before)
'poor old bums in' is the wordplay.
'poor' indicates an anagram.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'o'+'bums'+'in'='obumsin'
'obumsin' is an anagram of 'OMNIBUS'.
(Other definitions for omnibus that I've seen before include "collection of novels?" , "Large volume; means of transport" , "Collection of books reprinted in one volume" , "Comprehensive collection of books" , "Passenger-carrier" .)