Russian sleigh (6)
I believe the answer is:
'russian sleigh' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for troika that I've seen before include "Group of three" , "Russian three-horse coach" , "Russian vehicle drawn by three horses abreast" , "Russian three-horse carriage" , "Three-horse Russian vehicle" .)