Long period certain for deletion (7)
I believe the answer is:
erasure
'deletion' is the definition.
(erasure is a kind of deletion)
'long period certain' is the wordplay.
'long period' becomes 'era' (I've seen this before**).
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'era'+'sure'='ERASURE'
'for' acts as a link.
(Other definitions for erasure that I've seen before include "clearing of tape" , "Band with Andy Bell and Vince Clarke, or wipe-out" , "Wiping out - English pop duo" , "Rubbing out - English synthpop duo" , "group from the 80s" .)