Eternally is always extra (8)
I believe the answer is:
evermore
'eternally' is the definition.
(I've seen this before)
'always extra' is the wordplay.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'extra' becomes 'more' (I've seen this before).
'ever'+'more'='EVERMORE'
'is' acts as a link.
(Other definitions for evermore that I've seen before include "Always and without end" , "For which, permanently" , "Eternally" , "Always, forever" , "To all eternity" .)