One in the eye ultimately for rude girl (6)
I believe the answer is:
retina
'one in the eye' is the definition.
('retina' is part of 'eye')
'ultimately for rude girl' is the wordplay.
'ultimately' suggests the final letters.
'girl' becomes 'tina' (**).
The final letters of 'for rude' are 're'.
're'+'tina'='RETINA'
(Other definitions for retina that I've seen before include "Part of the eyeball" , "What's behind the humour" , "Light-receptive part of eye" , "Ocular membrane" , "It helps one to see" .)