Upsetting bulletin about old soldier (7)
I believe the answer is:
trooper
'soldier' is the definition.
(trooper is a kind of soldier)
'upsetting bulletin about old' is the wordplay.
'upsetting' says the letters should be written backwards.
'bulletin' becomes 'report' (I've seen this before).
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'report' going around 'o' is 'repoort'.
'repoort' back-to-front is 'TROOPER'.
(Other definitions for trooper that I've seen before include "ship" , "Cavalryman" , "Cavalry soldier" , "Cavalry 21" , "Soldier, proverbial swearer" .)