British and American transport (3)
I believe the answer is:
bus
'transport' is the definition.
(busing is a kind of transporting)
'british and american' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'and' means one lot of letters go next to another.
'american' becomes 'US' (abbreviation for United States).
'b'+'us'='BUS'
(Other definitions for bus that I've seen before include "People carrier" , "eg Double-decker" , "Coach" , "Road vehicle able to carry many passengers" , "Large passenger vehicle - small business!" .)