Russian leader is one at front (4)
I believe the answer is:
ivan
'russian' is the definition.
(I've seen this before)
'leader is one at front' is the wordplay.
'leader' indicates taking the first letters.
'one at' is a charade indicator (letters next to each other).
'front' becomes 'van' (I've seen this before).
The initial letter of 'is' is 'i'.
'i'+'van'='IVAN'
(Other definitions for ivan that I've seen before include "Name of six Russian tzars" , "Vain (anag) - Tsar of Russia" , "Russian tsar called 'the Terrible'" , "Fellow" , "'Terrible' emperor of Russia" .)