Authorisation for a significant assignment (10)
I believe the answer is:
permission
'authorisation' is the definition.
(I know that permission is a type of authorisation)
'a significant assignment' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'significant assignment' becomes 'mission' (mission is a kind of assignment. I am not sure about the 'significant' bit.).
'per'+'mission'='PERMISSION'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for permission that I've seen before include "Allowance" , "Leave" , "Visa, eg" , "Green light" , "Consent, authorisation" .)