Second-class American form of transport (3)
I believe the answer is:
bus
'form of transport' is the definition.
'bus' can be an answer for 'transport' (bus is a kind of transport). I am unsure of the 'form of' bit.
'second-class american' is the wordplay.
'second-class' becomes 'b' (**).
'american' becomes 'us' (abbreviation for United States).
'b'+'us'='BUS'
(Other definitions for bus that I've seen before include "Public service vehicle" , "Mode of public transport" , "Public transport vehicle" , "Charabanc" , "Coach" .)