Statement backed imprisoning old soldier (7)
I believe the answer is:
trooper
'soldier' is the definition.
(I know that Trooper is a type of soldier)
I don't understand the rest of the clue.
(Other definitions for trooper that I've seen before include "Cavalryman" , "Private soldier" , "Soldier, proverbial swearer" , "ship" , "Proverbially swearing soldier" .)