Old keg treated with new understanding (9)
I believe the answer is:
knowledge
'understanding' is the definition.
(I've seen this before)
'old keg treated with new' is the wordplay.
'treated with' indicates anagramming the letters.
'old'+'keg'='oldkeg'
'oldkeg' anagrammed with 'new' makes 'KNOWLEDGE'.
(Other definitions for knowledge that I've seen before include "Information gained by learning" , "Information - erudition" , "The result of perception, learning and reasoning" , "Grasp" , "Awareness - erudition" .)