Vessel of ale containing a third of keg (6)
I believe the answer is:
'vessel' is the definition.
(beaker is a kind of vessel)
'ale containing a third of keg' is the wordplay.
'ale' becomes 'beer' (I've seen this before).
'containing' indicates putting letters inside.
'third of keg' becomes 'k' (I can't explain this - if you can you should give a lot more credence to this answer).
'beer' placed around 'ak' is 'BEAKER'.
'of' is the link.
Can you help me to learn more?
(Other definitions for beaker that I've seen before include "Flatbottomed drinking cup", "Wide-mouthed cup", "Plastic drinking vessel", "Glass vessel", "Mug - large glass".)