Lack of moderation (6)
I believe the answer is:
excess
'lack of moderation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for excess that I've seen before include "Lack of moderation, in drinking say" , "Subjugate by force" , "Surfeit; overindulgence" , "Too much, surplus" , "Immoderation as in eating, too much" .)