Historically a family of maltreated mice - about 501 (6)
I believe the answer is:
medici
'historically a family' is the definition.
I can't judge whether this defines the answer.
'maltreated mice about 501' is the wordplay.
'maltreated' is an anagram indicator.
'about' is an insertion indicator.
'501' becomes 'di' (501 in Roman numerals).
'mice' with letters rearranged gives 'meci'.
'meci' placed around 'di' is 'MEDICI'.
'of' is the link.
Can you help me to learn more?
(Other definitions for medici that I've seen before include "Well-known European family" , "Name of great Florence family in 15-17 centuries" , "Name of great Florentine family 15-17th centuries" , "Famous Florentine family name" , "Powerful 15th-century family of bankers and merchants in Florence" .)