Is it the poor return in an unsuccessful winter that makes him swear? (7)
I believe the answer is:
trooper
'an unsuccessful winter that makes him swear?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'is it the poor return in' is the wordplay.
'is it' becomes 'ter' (I can't justify this - if you can you should believe this answer much more).
'return' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'poor' in reverse letter order is 'roop'.
'ter' going around 'roop' is 'TROOPER'.
Can you help me to learn more?
(Other definitions for trooper that I've seen before include "Cavalry 21" , "He swears" , "Soldier, proverbial swearer" , "ship" , "Cavalryman" .)