Locomotive one in Leeds crashing (6)
I believe the answer is:
diesel
'locomotive' is the definition.
(diesel locomotive is a kind of locomotive)
'one in leeds crashing' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'crashing' indicates an anagram.
'leeds' is an anagram of 'desel'.
'i' put within 'desel' is 'DIESEL'.
(Other definitions for diesel that I've seen before include "Motor fuel" , "Lorry fuel" , "Derv" , "Fuel used by heavy vehicles" , "Kind of engine" .)