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 subsidiary indication.
'a' becomes 'an'.
'little bit' becomes 'atom'.
'ultimately' suggests the final letters.
The final letter of 'grisly' is 'y'.
(Other definitions for anatomy that I've seen before include "Body structure", "Study of animal structures", "(Science of) physical structure of animals or plants", "Bodily structure of animal or plant", "Study of bodily structure", "Scientific study of bodily structure", "Study of body structure", "The scientific study of bodily structures", "Study of the structure of animals" and "Study of bodies".)