Walter's terrible idler (7)
I believe the answer is:
wastrel
'idler' is the definition.
(thesaurus)
'walter's terrible' is the wordplay.
'terrible' indicates anagramming the letters.
'walters' with letters rearranged gives 'WASTREL'.
(Other definitions for wastrel that I've seen before include "Good-for-nothing person" , "A useless, good for nothing fellow" , "Layabout" , "Spendthrift" , "Profligate" .)