Went through phase with Eno retrospective a very long time ago (5,3)
I believe the answer is:
stone age
'long time ago' is the definition.
(I've seen this in another clue)
'went through phase with eno retrospective a' is the wordplay.
I cannot quite see how this works, but
an anagram of 'eno' is 'one' which is located in the answer.
'phase' could be 'stage' (synonyms) and 'stage' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
'very' acts as a link.
Can you help me to learn more?
(Other definitions for stone age that I've seen before include "Prehistoric period" , "Goat seen (anag.)" , "Neolithic" , "before copper came in" , "Old time" .)