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 quite understand 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 found in the answer.
No letters remain.
This explanation may well be incorrect...
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" .)