One who took the lead in old Russia (4)
I believe the answer is:
ivan
'old russia' is the definition.
The definition suggests a singular noun which matches the answer.
'one who took the lead' is the wordplay.
'one who' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'took' says to put letters next to each other.
'the lead' becomes 'van' (vanguard is the leading military formation).
'i'+'van'='IVAN'
'in' is the link.
Can you help me to learn more?
(Other definitions for ivan that I've seen before include "man who had a terrible reputation" , "'Terrible' emperor of Russia" , "A Russian emperor, quite Terrible" , "Chap" , "Russian boy's name" .)