Lady that is part of orchestra (5)
I believe the answer is:
viola
I believe this is a double definition.
'lady' is the first definition.
'is part of orchestra' is the second definition.
(I've seen this before)
'that' acts as a link.
(Other definitions for viola that I've seen before include "Flower -- stringed instrument" , "Achievement" , "one used to bowing?" , "Bloomer" , "Large fiddle" .)