Ted, turning up outside the Post Office HQ (5)
I believe the answer is:
depot
'hq' is the definition.
(depot can mean a military headquarters)
'ted turning up outside the post office' is the wordplay.
'turning up' shows that the letters should be reversed in order.
'outside' is an insertion indicator (some letters go outside others).
'the post office' becomes 'po' (abbreviation).
'ted' backwards is 'det'.
'det' placed around 'po' is 'DEPOT'.
(Other definitions for depot that I've seen before include "Regimental HQ" , "Depository for goods" , "(Military) store" , "Storehouse, repository" , "Bus station in the US" .)