Is he packing up fighting? (5)
I believe the answer is:
boxer
'is he packing up fighting?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he packing up fighting?' is the wordplay.
'he' becomes 'Ber'.
'packing' means one lot of letters goes inside another (some letters are packed inside).
'up fighting?' becomes 'ox' (I am not sure about this - if you are sure you should believe this answer much more).
'ber' enclosing 'ox' is 'BOXER'.
Can you help me to learn more?
(Other definitions for boxer that I've seen before include "canine" , "Breed of stocky short-haired dog" , "one that's accustomed to fighting" , "Dog - fighter" , "Pug - dog" .)