Accidental loss of ale a keg contains (7)
I believe the answer is:
leakage
'accidental loss of ale a keg contains' is the definition.
I can't judge whether this definition defines the answer.
'loss of ale a keg contains' is the wordplay.
'loss' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'contains' is an insertion indicator.
'ale'+'keg'='alekeg'
'alekeg' is an anagram of 'leakge'.
'a' placed into 'leakge' is 'LEAKAGE'.
Can you help me to learn more?
(Other definitions for leakage that I've seen before include "Gradual escape" , "Amount seeping away" , "Unwanted discharge of fluid" , "Lake age (anag.)" , "Unwanted escape (eg of water or information)" .)