Run as one simply cannot do, on a half of beer (6)
I believe the answer is:
canter
'run' is the definition.
(I've seen this in another clue)
'do on a half of beer' is the wordplay.
'do on' becomes 'can' (I can't explain this - if you can you should give a lot more credence to this answer).
'a half of' suggests halving the letters.
'beer' becomes 'bitter' (bitter is a type of beer).
'bitter' halved is 'ter'.
'can'+'ter'='CANTER'
'as one simply cannot' 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 canter that I've seen before include "(Horse's) pace between a trot and a gallop" , "Horse gait quicker than 9 across" , "Lope" , "Gait between a trot and a gallop" , "Pace of a horse between trot and gallop" .)