Spoil line inserted in software program (4)
I believe the answer is:
blot
'spoil' is the definition.
(thesaurus)
'line inserted in software program' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'inserted in' indicates putting letters inside.
'software program' becomes 'bot' (bot can mean an autonomous computer programme).
'l' inserted into 'bot' is 'BLOT'.
(Other definitions for blot that I've seen before include "Don't do it to your copybook" , "Ink mark" , "Dry piece of writing" , "Soak up (ink); blemish" , "Stain left by spilled ink" .)