Many a girl is kind (5)
I believe the answer is:
class
'is kind' is the definition.
(I've seen this in another clue)
'many a girl' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for l00).
'a girl' becomes 'lass' (lass can informally mean a girl).
'c'+'lass'='CLASS'
(Other definitions for class that I've seen before include "sophistication" , "Kind" , "Body of students" , "26 [ORDER]" , "showing stylish excellence" .)