Concern public transport's in mess, losing millions (8)
I believe the answer is:
business
'concern' is the definition.
(concern can mean a business or company)
'public transport's in mess losing millions' is the wordplay.
'public transport's' becomes 'bus' (bus is a kind of public transport).
'losing' suggests deleting specific letters.
'millions' becomes 'm' (abbreviation).
'mess' with 'm' taken away is 'ess'.
'bus'+'in'+'ess'='BUSINESS'
(Other definitions for business that I've seen before include "Affair - matters to be attended to" , "See 1" , "Occupation; trade" , "Baby" , "Commerce, trade" .)