Science a little bit grisly, ultimately (7)
I believe the answer is:
anatomy
'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'.
'an'+'atom'+'y'='ANATOMY'
(Other definitions for anatomy that I've seen before include "Branch of biology" , "Study of animal structures" , "(Science of) physical structure of animals or plants" , "Bodily structure of animal or plant" , "Examining body" .)