A quick nap in the post office (6)
I believe the answer is:
presto
'a quick' is the definition.
(I've seen this before)
'nap in the post office' is the wordplay.
'nap' becomes 'rest' (I've seen this in another clue).
'in the' means one lot of letters goes inside another.
'post office' becomes 'po' (abbreviation).
'rest' placed into 'po' is 'PRESTO'.
(Other definitions for presto that I've seen before include "Hey . . . . ., says the magician (6)" , "at great speed..." , "Poster (anag.)" , "scored quickly" , "Hey, quickly for a magician" .)