Girl seen in lovely diamonds (5)
I believe the answer is:
lydia
'girl' is the definition.
(woman's name)
'in lovely diamonds' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LYDIA' can be found hidden inside 'lovely diamonds'.
'seen' acts as a link.
(Other definitions for lydia that I've seen before include "Woman" , "Old country" , "Daily it's a girl's name" , "Old kingdom" , "Roman province" .)