Spendthrift lets war break out (7)
I believe the answer is:
wastrel
'spendthrift' is the definition.
(I've seen this before)
'lets war break out' is the wordplay.
'break out' indicates anagramming the letters.
'lets'+'war'='letswar'
'letswar' with letters rearranged gives 'WASTREL'.
(Other definitions for wastrel that I've seen before include "Person who dissipates resources self-indulgently" , "A useless, good for nothing fellow" , "Ne'er-do-well" , "Good-for-nothing person" , "Useless person" .)