Pro at the bar on two beers, drinking gallons (5,5)
I believe the answer is:
legal eagle
'pro at the bar' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'on two beers drinking gallons' is the wordplay.
I cannot really see how this works, but
'on' could be 'leg' (cricket term, fielding positions) and 'leg' is located in the answer.
'gallons' could be 'g' (abbreviation) and 'g' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for legal eagle that I've seen before include "Bright lawyer (colloq.)" , "Brief" , "Astute lawyer" , "perhaps the best defence" .)