Protest about a king's means of transport
I believe the answer is:
railcar
'of transport' is the definition.
'railcar' can be an answer for 'transport' (railcar is a kind of transport). I am not sure about the 'of' bit.
'protest about a king's' is the subsidiary indication.
'protest' becomes 'rail'.
'about' becomes 'c' (abbreviation for 'circa'**).
'king' becomes 'r' (r is abbreviation for rex).
'rail'+'c'+'a'+'r'='railcar'
'means' is the link.
Can you help me to learn more?
(Other definitions for railcar that I've seen before include "Small train" , "coach" , "It's found on tracks" , "form of transport" , "Powered train unit" .)