Deletion is certain after a period of time (7)
I believe the answer is:
erasure
'deletion' is the definition.
(I know that erasure is a type of deletion)
'certain after a period of time' is the wordplay.
'certain' becomes 'sure' (synonyms).
'after' says to put letters next to each other.
'a period of time' becomes 'era' (era is a kind of period of time).
'sure' after 'era' is 'ERASURE'.
'is' acts as a link.
(Other definitions for erasure that I've seen before include "Effect of surface damage" , "Wiping out - English pop duo" , "Removal" , "Obliteration - rare use (anag)" , "Censorship" .)