He swears he's in the cavalry (7)
I believe the answer is:
trooper
'he swears' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for trooper that I've seen before include "Cavalry 21" , "Soldier, proverbial swearer" , "ship" , "Private soldier" , "He swears" .)