…by, when you spoil her (3)
I believe the answer is:
per
'by' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for per that I've seen before include "Preposition indicating by means of" , "Preposition meaning for each" , "According to" , "For each or every" .)