Sent material made an impression with staff (10,4)
I believe the answer is:
registered post
'staff' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?
'sent material made an impression' is the wordplay.
'sent' becomes 'registered' (registering is a kind of sending).
'material made an impression' becomes 'post' (I can't explain this - if you can you should give a lot more credence to this answer).
'registered'+'post'='REGISTERED POST'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for registered post that I've seen before include "Type of mail" , "important letters? - with 28a" , "Special form of delivery" , "sign for this?" , "Specially secure delivery" .)