Fellows suppressing wrong number, keeping one hidden for a long time (10)
I believe the answer is:
millennium
'a long time' is the definition.
(a millennium is a thousand years)
'fellows suppressing wrong number keeping one hidden' is the wordplay.
I cannot really understand how this works, but
'wrong' could be 'ill' (I've seen this in another clue) and 'ill' is located in the answer.
'fellows' could be 'men' (fellow can mean a man) and 'men' is located in the leftover letters.
'number' could be 'm' (M is an example) and 'm' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
The remaining letters 'nu' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for millennium that I've seen before include "1,000 years" , "Period of one thousand years" , "age" .)