One day she finishes the organ (3)
I believe the answer is:
eye
'organ' is the definition.
(eye is a kind of organ)
'one day she finishes' is the wordplay.
'finishes' suggests the final letters (I've seen 'finish' mean this).
The final letters of 'one day she' are 'EYE'.
'the' acts as a link.
(Other definitions for eye that I've seen before include "potato bud" , "Needle or lace hole" , "central spot" , "'Please your... and plague your heart'" , "20 [OBSERVE]" .)