Thing in use is dud, awfully mediocre (15)
I believe the answer is:
undistinguished
'mediocre' is the definition.
(thesaurus)
'thing in use is dud awfully' is the wordplay.
'awfully' indicates anagramming the letters.
'thing'+'in'+'use'+'is'+'dud'='thinginuseisdud'
'thinginuseisdud' anagrammed gives 'UNDISTINGUISHED'.
(Other definitions for undistinguished that I've seen before include "Ordinary, unexceptional" , "Leading ordinary lives" , "like some twins?" , "nothing special" .)