Having lost belly, able to drink a stout (3)
I believe the answer is:
fat
'stout' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for fat that I've seen before include "eg Lard" , "Excess body weight" , "Corpulent; little (chance)" , "Adipose tissue" , "As Bunter" .)