Deletion certain after long time (7)
I believe the answer is:
erasure
'deletion' is the definition.
(I know that erasure is a type of deletion)
'certain after long time' is the wordplay.
'certain' becomes 'sure' (synonyms).
'after' says to put letters next to each other.
'long time' becomes 'era' (a long period of time).
'sure' put after 'era' is 'ERASURE'.
(Other definitions for erasure that I've seen before include "Deletion by an act of expunging" , "clearing of tape" , "Effect of surface damage" , "group from the 80s" , "Obliteration - rare use (anag)" .)