Old transport company secures one ship (5)
I believe the answer is:
liner
'ship' is the definition.
(as in an ocean liner)
'old transport company secures one' is the wordplay.
'old transport company' becomes 'LNER' (British train company).
'secures' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'lner' enclosing 'i' is 'LINER'.
(Other definitions for liner that I've seen before include "Cosmetic; fine paintbrush" , "QE2, perhaps" , "eg QE2" , "Large passenger or cargo ship travelling on regular route" , "Passenger transport" .)