Promptly torn apart in the Post Office (6)
I believe the answer is:
pronto
'promptly' is the definition.
(I know that promptly can be written as pronto)
'torn apart in the post office' is the wordplay.
'apart' indicates an anagram.
'in the' means one lot of letters goes inside another.
'post office' becomes 'po' (abbreviation).
'torn' anagrammed gives 'ront'.
'ront' put inside 'po' is 'PRONTO'.
(Other definitions for pronto that I've seen before include "Right now, amigo" , "Right now, immediately" , "Right away, quickly" , "Quickly (sl.)" , "Quickly, amigo" .)