Works inane system for extremely long spells (9)
I believe the answer is:
millennia
'extremely long spells' is the definition.
The answer and definition can be both related to time as well as being plural nouns.
Maybe they are linked in a way I don't understand?
'works inane system' is the wordplay.
'works' becomes 'mill' (mill is a kind of works).
'system' indicates an anagram (another version or system with the same letters).
'inane' is an anagram of 'ennia'.
'mill'+'ennia'='MILLENNIA'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for millennia that I've seen before include "Thousand-year periods" , "Thousand year periods" , "Thousands of years" , "2000 years minimum" , "protracted periods" .)