Impertinent? I’m leaving as appropriate (9)
I believe the answer is:
pertinent
'appropriate' is the definition.
('pertinent' can be a synonym of 'appropriate')
'impertinent? i'm leaving' is the wordplay.
'leaving' is a deletion indicator.
'impertinent' with 'im' taken away is 'PERTINENT'.
'as' acts as a link.
(Other definitions for pertinent that I've seen before include "Apposite" , "Relevent, material" , "Relevant, to the point" , "fitting" , "Appropriate" .)