One who swears or turns into a drunkard (7)
I believe the answer is:
trooper
'one who swears' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'or turns into a drunkard' is the wordplay.
'turns' is an anagram indicator.
'into' means one lot of letters goes inside another.
'a drunkard' becomes 'toper' (I've seen this in other clues).
'or' with letters rearranged gives 'ro'.
'ro' going within 'toper' is 'TROOPER'.
Can you help me to learn more?
(Other definitions for trooper that I've seen before include "Cavalry 21" , "Proverbially swearing soldier" , "Cavalry soldier" , "Cavalryman" , "Soldier, proverbial swearer" .)