No, you needn't ask again (5)
I believe the answer is:
never
'needn't ask again' is the definition.
I can't judge whether this definition defines the answer.
'no you' is the wordplay.
'no' becomes 'n' (common abbreviation y/n).
'you' becomes 'ever' (I can't justify this - if you can you should give a lot more credence to this answer).
'n'+'ever'='NEVER'
Can you help me to learn more?
(Other definitions for never that I've seen before include "' - - Say - - Again': Bond film" , "Not in a month of Sundays" , "At no time in the past or future" , "Strong negative" , "On no account" .)