It's quickly torn out in the Post Office (6)
I believe the answer is:
pronto
'it's quickly' is the definition.
(I've seen this before)
'torn out in the post office' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'in the' is an insertion indicator.
'post office' becomes 'po' (abbreviation).
'torn' is an anagram of 'ront'.
'ront' going inside 'po' is 'PRONTO'.
(Other definitions for pronto that I've seen before include "Double quick!" , "Right now, amigo" , "Quick as you can, amigo" , "Right away, quickly" , "Quickly (sl.)" .)