Sad deranged idiot, snapping (13)
I believe the answer is:
disappointing
'sad' is the definition.
(thesaurus)
'deranged idiot snapping' is the wordplay.
'deranged' indicates anagramming the letters.
'idiot'+'snapping'='idiotsnapping'
'idiotsnapping' is an anagram of 'DISAPPOINTING'.
(Other definitions for disappointing that I've seen before include "Failing to satisfy" , "Not as good as one had hoped" , "Poor" .)