Walter's wild, being a spendthrift (7)
I believe the answer is:
'a spendthrift' is the definition.
(I've seen this before)
'walter's wild' is the wordplay.
'wild' indicates anagramming the letters.
'walters' with letters rearranged gives 'WASTREL'.
'being' is the link.
(Other definitions for wastrel that I've seen before include "Worthless person" , "A useless, good for nothing fellow" , "Spendthrift" , "Profligate" , "Ne'er-do-well" .)