Rotten Médoc, awfully pretentious (10)
I believe the answer is:
'rotten' is the definition.
'medoc awfully pretentious' is the wordplay.
'awfully' indicates an anagram.
'pretentious' becomes 'posed' (I am not sure about this - if you are sure you should believe this answer much more).
'medoc' is an anagram of 'decom'.
Can you help me to learn more?
(Other definitions for decomposed that I've seen before include "broken down" , "Decayed" .)