A seeming eternity (4)
I believe the answer is:
ages
'a seeming eternity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ages that I've seen before include "A long time (colloq.)" , "Yonks" , "Awfully long time" , "(What seems) a very long time" , "Donkey's years" .)