Cancellation of important date certain (7)
I believe the answer is:
erasure
'cancellation' is the definition.
(thesaurus)
'important date certain' is the wordplay.
'important date' becomes 'era' (I've seen this before).
'certain' becomes 'sure' (synonyms).
'era'+'sure'='ERASURE'
'of' acts as a link.
(Other definitions for erasure that I've seen before include "Deletion by an act of expunging" , "Obliteration of data" , "Process of rubbing out" , "Wiping out - English pop duo" , "Band with Andy Bell and Vince Clarke, or wipe-out" .)