'Time' certain to show deletion (7)
I believe the answer is:
erasure
'deletion' is the definition.
(erasure is a kind of deletion)
'time certain' is the wordplay.
'time' becomes 'era' (I've seen this before).
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'era'+'sure'='ERASURE'
'to show' acts as a link.
(Other definitions for erasure that I've seen before include "Removal" , "Process of rubbing out" , "clearing of tape" , "Rubbing out - English synthpop duo" , "Effect of surface damage" .)