Spoil just one in a series (4)
I believe the answer is:
ruin
'spoil' is the definition.
(I know that spoil is a more specific form of ruin)
'one in a series' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'a series' becomes 'run' (a run or sequence of things).
'i' put inside 'run' is 'RUIN'.
'just' acts as a link.
(Other definitions for ruin that I've seen before include "Catastrophe" , "Wrecked building" , "building now demolished" , "Mar, lay waste to" , "State of devastation and destruction" .)