Some upset constituents detain Nell impolitely for ages (9)
I believe the answer is:
millennia
'for ages' is the definition.
(I've seen this before)
'some upset constituents detain nell impolitely' is the wordplay.
'upset' is a reversal indicator.
'some' says the answer is hidden in the clue (some parts of the word are used).
'ainnellim' is hidden within 'constituents detain nell impolitely'.
'ainnellim' in reverse letter order is 'MILLENNIA'.
(Other definitions for millennia that I've seen before include "Could seem an interminable time" , "Spans of 1,000 years" , "protracted periods" , "Thousand year periods" , "Periods of 1000 years" .)