Cavalryman returned report about nothing (7)
I believe the answer is:
trooper
'cavalryman' is the definition.
(I know that cavalryman can be written as trooper)
'returned report about nothing' is the wordplay.
'returned' says the letters should be written in reverse.
'about' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'report' placed around 'o' is 'repoort'.
'repoort' written backwards gives 'TROOPER'.
(Other definitions for trooper that I've seen before include "Soldier, proverbial swearer" , "Cavalryman" , "Cavalry 21" , "He swears" , "ship" .)