Sport in Post Office, behold! (4)
I believe the answer is:
polo
'sport' is the definition.
(polo is a kind of sport)
'post office behold' is the wordplay.
'post office' becomes 'po'.
'behold' becomes 'lo'.
'po'+'lo'='POLO'
'in' acts as a link.
(Other definitions for polo that I've seen before include "Game; Venetian explorer" , "Game played on horseback or in pool" , "Team game played with horses" , "Hockey on horseback" , "Game; Italian explorer" .)