Quick to stop working in the post office (6)
I believe the answer is:
presto
'quick' is the definition.
(I've seen this before)
'stop working in the post office' is the wordplay.
'stop working' becomes 'rest' (I've seen this in another clue).
'in the' indicates putting letters inside.
'post office' becomes 'po' (abbreviation).
'rest' going into 'po' is 'PRESTO'.
'to' is the link.
(Other definitions for presto that I've seen before include "hey......,cries the magician" , "29 [ADAGIO]? On the contrary" , "Hey, it's a magician's word" , "To be played quickly" , "quickly in bars" .)