Plain card containing new authorisation (9)
I believe the answer is:
clearance
'authorisation' is the definition.
(clearance is a kind of authorisation)
'plain card containing new' is the wordplay.
'plain' becomes 'clear' (similar in meaning).
'card' becomes 'ace' (ace is a type of playing card).
'containing' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'ace' enclosing 'n' is 'ance'.
'clear'+'ance'='CLEARANCE'
(Other definitions for clearance that I've seen before include "Removing ball from defensive area" , "Official authorisation" , "Go-ahead" , "Eviction from lands" , "OK" .)