One in Leeds replaced engine driver (6)
I believe the answer is:
diesel
'engine driver' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'one in leeds replaced' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'replaced' indicates anagramming the letters.
'leeds' anagrammed gives 'desel'.
'i' placed inside 'desel' is 'DIESEL'.
Can you help me to learn more?
(Other definitions for diesel that I've seen before include "Rudolf __ , French-born German engine inventor, d.1913" , "Type of engine" , "Vehicle fuel" , "Train perhaps" , "Common fuel for heavy vehicles" .)