A note left breaking a date can make one happy! (6)
I believe the answer is:
elated
'one happy' is the definition.
(I've seen this before)
'a note left breaking a date' is the wordplay.
'a note' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'left' says the letters should be written in reverse.
'breaking' is an anagram indicator.
'le' in reverse letter order is 'el'.
'date' is an anagram of 'ated'.
'el'+'ated'='ELATED'
'can make' is the link.
Can you help me to learn more?
(Other definitions for elated that I've seen before include "In an enraptured state" , "Jubilant, overjoyed" , "Thrilled" , "In high spirits, very pleased" , "In very high spirits, really chuffed" .)