Dear Sir, this editorial's extraordinarily pretentious, first of all (5)
I believe the answer is:
steep
'dear' is the definition.
(thesaurus)
'sir this editorial's extraordinarily pretentious first of all' is the wordplay.
'first of all' suggests taking the first letters.
The initial letters of 'sir this editorials extraordinarily pretentious' is 'STEEP'.
(Other definitions for steep that I've seen before include "Without the bounds of moderation" , "Rapidly rising" , "Soak thoroughly - precipitous" , "Sharply inclined" , "Hard for cycling" .)