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 quite 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 understand.
This explanation may well be incorrect...
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" .)