Philosopher terribly inane for long periods (9)
I believe the answer is:
millennia
'long periods' is the definition.
'millennia' can be an answer for 'periods' (millennium is a kind of period). I'm unsure of the 'long' bit.
'philosopher terribly inane' is the wordplay.
'philosopher' becomes 'mill' (John Stuart Mill).
'terribly' indicates anagramming the letters.
'inane' is an anagram of 'ennia'.
'mill'+'ennia'='MILLENNIA'
'for' is the link.
Can you help me to learn more?
(Other definitions for millennia that I've seen before include "Thousands of years" , "protracted periods" , "Could seem an interminable time" , "ages" , "Spans of 1,000 years" .)