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' is a charade indicator (letters next to each other).
'a half of' suggests halving the letters.
'beer' cut in half is 'er'.
'top'+'er'='TOPER'
'at his' is the link.
I am not very happy about this link. 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" .)