Show worker in post office (5)
I believe the answer is:
panto
'show' is the definition.
(pantomime)
'worker in post office' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'in' is an insertion indicator.
'post office' becomes 'po' (abbreviation).
'ant' inserted into 'po' is 'PANTO'.
(Other definitions for panto that I've seen before include "type of show" , "Cinderella or Puss in Boots, say?" , "Christmas entertainment (abbr.)" , "Slapstick theatre for children (abbr)" , "Trad. Christmas fun (abbr.)" .)