Spoils things when one runs out (5)
I believe the answer is:
ruins
'spoils things' is the definition.
'ruins' can be an answer for 'spoils' (ruining is a kind of spoiling). I'm not sure about the 'things' bit.
'one runs out' is the wordplay.
'one' becomes 'i' (Roman numeral).
'out' indicates putting letters inside.
'i' put within 'runs' is 'RUINS'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for ruins that I've seen before include "Wrecked buildings" , "Damaged remains (of building)" , "Broken-down remnants" , "dilapidated buildings" , "Damages beyond repair" .)