Drinker at his best with a half of beer (5)
I believe the answer is:
toper
'drinker' is the definition.
(synonyms)
'best with a half of beer' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'with' means one lot of letters go next to another.
'a half of' indicates taking half.
'beer' cut in half is 'er'.
'top'+'er'='TOPER'
'at his' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for toper that I've seen before include "Someone with a drink problem" , "Regular drinker" , "Serious drinker" , "Drunkard" , "Heavy drinker" .)