Rubbing out for a long time, certainly (7)
I believe the answer is:
erasure
'rubbing out' is the definition.
(I've seen this before)
'long time certainly' is the wordplay.
'long time' becomes 'era' (a long period of time).
'certainly' becomes 'sure' (synonyms).
'era'+'sure'='ERASURE'
'for a' is the link.
(Other definitions for erasure that I've seen before include "Wiping out - English pop duo" , "Band with Andy Bell and Vince Clarke, or wipe-out" , "clearing of tape" , "Rubbing out - English synthpop duo" , "Obliteration - rare use (anag)" .)