Important mission regenerating assembly line abandoned (7)
I believe the answer is:
embassy
'important mission' is the definition.
'embassy' can be an answer for 'mission' (I've seen this before). I'm not certain of the 'important' bit.
'regenerating assembly line abandoned' is the wordplay.
'regenerating' indicates an anagram.
'line' becomes 'l' (used when specifying particular lines from a poem).
'abandoned' suggests deleting specific letters.
'assembly' with 'l' taken away is 'assemby'.
'assemby' with letters rearranged gives 'EMBASSY'.
Can you help me to learn more?
(Other definitions for embassy that I've seen before include "foreign mission abroad" , "Diplomatic building for ambassadors" , "Ambassador's office" , "Legation" , "Ambassador's residence" .)