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' says to put letters next to each other.
'a half of' means to take half the letters.
'beer' with half the letters taken is 'er'.
'top'+'er'='TOPER'
'at his' is the link.
This may not be right. Some or all of it may be part of 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" .)