Secret police confiscated uranium from guest beside a post office (7)
I believe the answer is:
gestapo
'secret police' is the definition.
(Nazi secret police)
'uranium from guest beside a post office' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'from' is a deletion indicator.
'beside' is a charade indicator (letters next to each other) (I've seen this in other clues).
'post office' becomes 'po' (abbreviation).
'guest' with 'u' removed is 'gest'.
'gest'+'a'+'po'='GESTAPO'
'confiscated' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for gestapo that I've seen before include "Men in black" , "Nazi Germany's secret state police" , "Postage to Nazi secret police" , "war force" , "Geheime Staatspolizei" .)