Large mouthful of punch (4)
I believe the answer is:
slug
'punch' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for slug that I've seen before include "Wallop" , "Slimy gastropod" , "ammunition?" , "Slimy fellow in bed" , "Projectile fired from a gun" .)