Horseman spots revolutionary American taken inside (6)
I believe the answer is:
hussar
'horseman' is the definition.
(historical term for a horseman in Europe)
'spots revolutionary american taken inside' is the wordplay.
'spots' becomes 'rash' (as in skin irritation).
'revolutionary' is a reversal indicator.
'american' becomes 'us' (abbreviation for United States).
'taken inside' means one lot of letters goes inside another.
'rash' written backwards gives 'hsar'.
'hsar' going around 'us' is 'HUSSAR'.
(Other definitions for hussar that I've seen before include "Cavalryman, originally Hungarian" , "Soldier in a light cavalry regiment" , "Light cavalryman" , "Hungarian soldier" , "Member of a light cavalry regiment" .)