Small warship has rust: order reversed for large transporter (8)
I believe the answer is:
motorbus
'large transporter' is the definition.
'small warship has rust order reversed' is the wordplay.
'small warship' becomes 'sub' (short for submarine, a submersible warship; 'small' suggests a short form).
'has' means one lot of letters go next to another.
'rust' becomes 'rot'.
'order' becomes 'om' (Order of Merit).
'reversed' shows that the letters should be reversed in order.
'sub'+'rot'+'om'='subrotom'
'subrotom' back-to-front is 'MOTORBUS'.
'for' acts as a link.
(Another definition for motorbus that I've seen is " Passenger vehicle".)