Impetuous revolutionary captures useless soldier on horse (6)
I believe the answer is:
hussar
'soldier on horse' is the definition.
'hussar' can be an answer for 'soldier' (hussar is a kind of soldier). I'm not sure about the 'on horse' bit.
'impetuous revolutionary captures useless' is the wordplay.
'impetuous' becomes 'rash' (I've seen this before).
'revolutionary' is a reversal indicator (letters revolve around).
'captures' means one lot of letters goes inside another.
'useless' becomes 'us' (abbreviation for unserviceable).
'rash' in reverse letter order is 'hsar'.
'hsar' enclosing 'us' is 'HUSSAR'.
Can you help me to learn more?
(Other definitions for hussar that I've seen before include "Cavalryman, originally Hungarian" , "Member of a light cavalry regiment" , "Soldier in a light cavalry regiment" , "Horseman" , "Light cavalryman" .)