Science a little bit grisly, ultimately (7)
I believe the answer is:
Here is my best explanation:
'science' is the definition.
(anatomy is a kind of science)
'a little bit grisly ultimately' is the wordplay.
'a' becomes 'an'.
'little bit' becomes 'atom'.
'ultimately' indicates one should take the final letters.
The last letter of 'grisly' is 'y'.
(Other definitions for anatomy that I've seen before include "Examining body", "Bodily structure of animal or plant", "Study of animal structures", "Study of body structure", "Science of bodily structure", "Scientific study of bodily structure", "(Science of) physical structure of animals or plants", "Study of bodies", "Study of bodily structure" and "Physical structure".)