Unfinished station on motorway held up by motorised transport (5)
I believe the answer is:
moped
'transport' is the definition.
(moped is a kind of transport)
'unfinished station on motorway' is the wordplay.
'unfinished station' becomes 'oped' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'motorway' becomes 'm'.
'oped' after 'm' is 'MOPED'.
'held up by motorised' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for moped that I've seen before include "means of transport" , "Light motor cycle" , "Vehicle - was blue" , "Light motorbike" , "Wandered about listlessly" .)