All others in post office can be very quick (6)
I believe the answer is:
presto
'very quick' is the definition.
'all others in post office' is the wordplay.
'all others' becomes 'rest' (as in the rest of a group).
'in' indicates putting letters inside.
'post office' becomes 'po'.
'rest' placed inside 'po' is 'PRESTO'.
'can be' acts as a link.
(Other definitions for presto that I've seen before include "fast to the maximum" , "quick to score" , "Poster (anag.)" , "In quick tempo (mus.)" , "quickly in bars" .)