A note left breaking a date can make one happy! (6)
I believe the answer is:
Here is my best explanation:
'one happy' is the definition.
(I've seen this before)
'a note left breaking a date can make' is the subsidiary indication.
'a note' becomes 'e' (musical note**).
'left' becomes 'l' (common abbreviation).
'breaking' is an insertion indicator.
'can make' indicates anagramming the letters.
'date' with letters rearranged gives 'ated'.
'l' placed into 'ated' is 'lated'.
(Other definitions for elated that I've seen before include "In an enraptured state", "Excited and jubilant", "Happy and jubilant", "On cloud nine", "Overjoyed, in high spirits", "Ecstatic", "Overjoyed, happy and excited", "In very high spirits, really chuffed", "Thrilled" and "Euphoric".)