Deletion certain to be seen after a long time (7)
I believe the answer is:
erasure
'deletion' is the definition.
(I know that erasure is a type of deletion)
'certain to be seen after a long time' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to be seen after' means one lot of letters go next to another.
'a long time' becomes 'era' (a long period of time).
'sure' after 'era' is 'ERASURE'.
(Other definitions for erasure that I've seen before include "Process of rubbing out" , "Band with Andy Bell and Vince Clarke, or wipe-out" , "Deletion by an act of expunging" , "Rubbing out - English synthpop duo" , "Obliteration - rare use (anag)" .)