Many a girl shows spite (6)
I believe the answer is:
malice
'spite' is the definition.
(synonyms)
'many a girl' is the wordplay.
'many' becomes 'M' (Roman numeral for a thousand).
'girl' becomes 'lice' (I can't justify this - if you can you should believe this answer much more).
'm'+'a'+'lice'='MALICE'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for malice that I've seen before include "Hatred" , "evil intent" , "Spite and rancour" , "'Ill-will, spite (6)'" , "Spiteful, animosity, sometimes a forethought" .)