A musical embellishment for the girl to observe (5,4)
I believe the answer is:
grace note
'a musical embellishment' is the definition.
(I've seen this before)
'girl to observe' is the wordplay.
'girl' becomes 'grace' ().
'to observe' becomes 'note' (synonyms).
'grace'+'note'='GRACE NOTE'
'for the' is the link.
(Other definitions for grace note that I've seen before include "musical embellishment" , "Perhaps Nimrod's embellishment" .)