A medal to brighten up that dull uniform? (10)
I believe the answer is:
decoration
This clue could be a double definition.
'a medal to brighten up that dull' is the first definition.
'decoration' can be an answer for 'a medal' (synonyms). I'm not sure about the rest of the definition.
'uniform?' is the second definition.
(thesaurus)
Can you help me to learn more?
(Other definitions for decoration that I've seen before include "award" , "Flags put out" , "Star" , "Bunting (in honour of a brave man?)" , "Ornaments - medal" .)