Wife alters plan for spendthrift (7)
I believe the answer is:
wastrel
'for spendthrift' is the definition.
(I've seen this before)
'wife alters plan' is the wordplay.
'wife' becomes 'w'.
'plan' indicates anagramming the letters (plan can mean organise).
'alters' is an anagram of 'astrel'.
'w'+'astrel'='WASTREL'
(Other definitions for wastrel that I've seen before include "Useless person" , "Profligate" , "Prodigal" , "Person who dissipates resources self-indulgently" , "Layabout" .)