Horribly inane foundation for philosopher's golden ages (9)
I believe the answer is:
millennia
'ages' is the definition.
(I've seen this before)
'horribly inane foundation for philosopher's golden' is the wordplay.
I cannot really see how this works, but
an anagram of 'inane' is 'ennia' which is present in the answer.
'philosopher' could be 'mill' (John Stuart Mill) and 'mill' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for millennia that I've seen before include "2000 years minimum" , "protracted periods" , "Spans of 1,000 years" , "Thousand year periods" , "Thousands of years" .)