In age, certain obliteration (7)
I believe the answer is:
erasure
'obliteration' is the definition.
(I've seen this before)
'in age certain' is the wordplay.
'in age' becomes 'era' (both can mean a long time. I am not sure about the 'in' bit.).
'certain' becomes 'sure' (synonyms).
'era'+'sure'='ERASURE'
(Other definitions for erasure that I've seen before include "Censorship" , "Process of rubbing out" , "Band with Andy Bell and Vince Clarke, or wipe-out" , "Removal" , "Effect of surface damage" .)