One head backing transport company (8)
I believe the answer is:
business
'company' is the definition.
(thesaurus)
'one head backing transport' is the wordplay.
'one' becomes 'i' (Roman numeral).
'head' becomes 'ness' (ness is a headland**).
'backing' means one lot of letters go next to another.
'transport' becomes 'bus' (bus is a kind of transport).
'i'+'ness'='iness'
'iness' put after 'bus' is 'BUSINESS'.
(Other definitions for business that I've seen before include "Concern; commerce" , "Commercial or industrial enterprise" , "See 1" , "complicated problem" , "Occupation; trade" .)