Complain about the transport? (4)
I believe the answer is:
rail
This might be a double definition.
'complain about' is the first definition.
'rail' can be an answer for 'complain' (railing is a kind of complaining). I'm not certain of the 'about' bit.
'transport?' is the second definition.
(I know that rail is a type of transport)
'the' is the link.
Can you help me to learn more?
(Other definitions for rail that I've seen before include "Supported bar - inveigh" , "wader" , "Part of train track" , "Line" , "Safety bar; bird" .)