Place bedecked in unalloyed imperial colour (6)
I believe the answer is:
purple
'imperial colour' is the definition.
(colour associated with Roman emperors)
'place bedecked in unalloyed' is the wordplay.
'place' becomes 'pl' (abbreviation used in postal addresses).
'bedecked in' indicates putting letters inside.
'unalloyed' becomes 'pure' (similar in meaning).
'pl' placed within 'pure' is 'PURPLE'.
(Other definitions for purple that I've seen before include "Red and blue mixed" , "Colour between 14dn and blue" , "Red and blue mix" , "Colour for aristocracy or emperors" , "Red-blue mixture" .)