Many a girl's spite (6)
I believe the answer is:
malice
'spite' is the definition.
(I know that spite can be written as malice)
'many a girl's' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'girl' becomes 'lice' (I can't justify this - if you can you should give a lot more credence to this answer).
'm'+'a'+'lice'='MALICE'
Can you help me to learn more?
(Other definitions for malice that I've seen before include "'Ill-will, spite (6)'" , "evil intent" , "Wrongful intention" , "Desire to harm others, may be aforethought" , "Spiteful, animosity, sometimes a forethought" .)