Note nine mull about for a long time (10)
I believe the answer is:
millennium
'a long time' is the definition.
(a millennium is a thousand years)
'note nine mull about' is the wordplay.
'note' becomes 'mi' (as in 'do re mi').
'about' indicates an anagram.
'nine'+'mull'='ninemull'
'ninemull' anagrammed gives 'llennium'.
'mi'+'llennium'='MILLENNIUM'
'for' acts as a link.
(Other definitions for millennium that I've seen before include "1,000 years" , "Period of one thousand years" , "age" .)