Cancel important dates with sweetheart (5)
I believe the answer is:
erase
'cancel' is the definition.
(I know that erase is a more specific form of cancel)
'important dates with sweetheart' is the wordplay.
'important dates' becomes 'eras' (I've seen this in another clue).
'with' says to put letters next to each other.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'eras'+'e'='ERASE'
(Other definitions for erase that I've seen before include "To remove all trace" , "Kill" , "Destroy recording on audio or video tape" , "Rub out, expunge" , "Blank out" .)