Assignment - pursuing keys for entry (9)
I believe the answer is:
admission
'entry' is the definition.
(I know that admission is a type of entry)
'assignment pursuing keys for' is the wordplay.
'assignment' becomes 'mission' (mission is a kind of assignment).
'pursuing' says to put letters next to each other.
'keys for' becomes 'ad' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mission' after 'ad' is 'ADMISSION'.
Can you help me to learn more?
(Other definitions for admission that I've seen before include "guilty revelation" , "Disclosure" , "Letting in - letting out" , "Statement acknowledging the truth of something" , "Acknowledgment of crime" .)