Main road through most of Russian city that is in pieces (6)
I believe the answer is:
mosaic
'in pieces' is the definition.
'mosaic' can be an answer for 'pieces' (I have seen 'Little pieces' mean 'mosaic' so perhaps 'pieces' could also mean 'mosaic'). I am unsure of the 'in' bit.
'main road through most of russian city that is' is the wordplay.
'main road' becomes 'AI' (resembles Am, major UK road).
'through' indicates putting letters inside (inserted letters go through the word).
'most of russian city' becomes 'mo' (I can't explain this - if you can you should give a lot more credence to this answer).
'that is' becomes 'sc' (abbrv. for Latin scilicet; 'namely').
'mo'+'sc'='mosc'
'ai' placed within 'mosc' is 'MOSAIC'.
Can you help me to learn more?
(Other definitions for mosaic that I've seen before include "Relating to Moses" , "Pattern of fragments" , "Design of many small pieces" , "Small pieces fitted together" , "Picture made up of coloured fragments" .)