A quick breather in the post office (6)
I believe the answer is:
presto
'a quick' is the definition.
(I've seen this before)
'breather in the post office' is the wordplay.
'breather' becomes 'rest' (I've seen this before).
'in the' is an insertion indicator.
'post office' becomes 'po' (abbreviation).
'rest' inserted into 'po' is 'PRESTO'.
(Other definitions for presto that I've seen before include "In quick tempo (mus.)" , "Very fast (mus.)" , "rapidly" , "Poster (anag.)" , "Moving quickly" .)