Does he swear to be a good soldier? (7)
I believe the answer is:
trooper
'soldier?' is the definition.
(trooper is a kind of soldier)
'does he swear to be a good' is the wordplay.
I cannot really understand how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trooper that I've seen before include "Soldier, proverbial swearer" , "He swears" , "Cavalryman" , "ship" , "Private soldier" .)