'Purple lady' in silly diamonds (5)
I believe the answer is:
lydia
'purple lady' is the definition.
(Lydia, the Woman of Purple, biblical figure)
'in silly diamonds' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LYDIA' is hidden in the letters of 'silly diamonds'.
(Other definitions for lydia that I've seen before include "girl" , "Old kingdom" , "Roman province" , "Female" , "Daily it's a girl's name" .)