Walter's turned into a spendthrift (7)
I believe the answer is:
'a spendthrift' is the definition.
(I've seen this in another clue)
'walter's turned' is the wordplay.
'turned' indicates anagramming the letters.
'walters' anagrammed gives 'WASTREL'.
'into' is the link.
(Other definitions for wastrel that I've seen before include "Worthless person" , "Ne'er-do-well" , "Person who dissipates resources self-indulgently" , "Prodigal" , "Layabout" .)