In passenger transport, it's not a big thing (7)
I believe the answer is:
minibus
'not a big thing' is the definition.
I can't tell whether this definition defines the answer.
'in passenger transport' is the wordplay.
'in passenger' becomes 'mini' (I can't justify this - if you can you should believe this answer much more).
'transport' becomes 'bus' (busing is a kind of transporting).
'mini'+'bus'='MINIBUS'
'it's' is the link.
Can you help me to learn more?
(Other definitions for minibus that I've seen before include "Small multi-seater vehicle" , "Small coach" , "Vehicle for up to, say, 15 passengers" , "form of transport" , "transport for children?" .)