Presumably he packs a punch (5)
I believe the answer is:
boxer
'presumably he packs a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'punch' is the wordplay.
I cannot really see how this works, but
'punch' could be 'box' (I've seen this before) and 'box' is found in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for boxer that I've seen before include "one that's accustomed to fighting" , "canine" , "Tyson, perhaps - type of dog" , "Pug - dog" , "Pugilist - dog" .)