Vessel of ale containing a third of keg (6)
I believe the answer is:
beaker
'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).
'a'+'k'='ak'
'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 "Plastic cup without a handle" , "Plastic mug" , "Drink container" , "Chemistry lab vessel" , "Mug - large glass" .)