Unspecified guy came back in, awfully nude! (7)
I believe the answer is:
unnamed
'unspecified' is the definition.
(thesaurus)
'guy came back in awfully nude' is the wordplay.
'guy' becomes 'man' (guy can informally mean a man).
'came back' says the letters should be written in reverse.
'in' is an insertion indicator.
'awfully' indicates an anagram.
'man' back-to-front is 'nam'.
'nude' with letters rearranged gives 'uned'.
'nam' placed inside 'uned' is 'UNNAMED'.
(Other definitions for unnamed that I've seen before include "anon" , "Anonymous < nark wishes to be ____" , "not placed in speech" , "Nude man (anag) - not identified" , "Not identified in mundane way" .)