Soldier from America infiltrating Soviet Union to the east of Hungary (6)
I believe the answer is:
hussar
'soldier' is the definition.
(hussar is a kind of soldier)
'america infiltrating soviet union to the east of hungary' is the wordplay.
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
'infiltrating' is an insertion indicator.
'soviet union' becomes 'ussr' ('USSR' can be a synonym of 'soviet union').
'to the east of' is a charade indicator (letters next to each other).
'hungary' becomes 'h'.
'a' going into 'ussr' is 'ussar'.
'ussar' after 'h' is 'HUSSAR'.
'from' is the link.
(Other definitions for hussar that I've seen before include "Member of a light cavalry regiment" , "one often up on a charge" , "Cavalryman, originally Hungarian" , "Early Hungarian cavalry man" , "Hungarian soldier" .)