Initially attenuated, grew ever such a long time (4)
I believe the answer is:
ages
'a long time' is the definition.
(ages can mean a very long time)
'initially attenuated grew ever such' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'attenuated grew ever such' is 'AGES'.
(Other definitions for ages that I've seen before include "A long time (colloq.)" , "... of Man, according to WB Yeats" , "Sage (anag)" , "Grows old like sage" , "Epochs" .)