Alcoholic upset about a glass of beer (5)
I believe the answer is:
toper
'alcoholic' is the definition.
(I've seen this before)
'upset about a glass of beer' is the wordplay.
'upset' says the letters should be written in reverse.
'about' becomes 're' ('re' means regarding or about).
'a glass of beer' becomes 'pot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
're'+'pot'='repot'
'repot' written backwards gives 'TOPER'.
Can you help me to learn more?
(Other definitions for toper that I've seen before include "Alcoholic" , "Tippler" , "Trope for habitual drinker" , "boozer" , "Serious drinker" .)