Trained ace, this writer's one who manages paper! (5)
I believe the answer is:
aimed
'trained' is the definition.
('aim' can be a synonym of 'train')
'ace this writer's one who manages paper' is the wordplay.
'ace' becomes 'a' (abbreviation).
'this writer's' becomes 'I'm' (the person writing the crossword clues).
'one who manages paper' becomes 'ed' (editor).
'a'+'im'+'ed'='AIMED'
(Other definitions for aimed that I've seen before include "Directed, targeted" , "trained" , "Focused on as target" , "Media (anag)" , "Tried; pointed" .)