He'll swear the porter's upset about nothing! (7)
I believe the answer is:
trooper
'he'll swear' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'porter's upset about nothing' is the wordplay.
'upset' is an anagram indicator.
'about' is an insertion indicator.
'nothing' becomes 'O' (looks like zero - 0).
'porter' with letters rearranged gives 'troper'.
'troper' enclosing 'o' is 'TROOPER'.
'the' is the link.
Can you help me to learn more?
(Other definitions for trooper that I've seen before include "ship" , "Proverbially swearing soldier" , "Cavalryman" , "Soldier, proverbial swearer" , "He swears" .)