Ultimately keep to leaner dark meat (4)
I believe the answer is:
pork
'meat' is the definition.
(pork is a kind of meat)
'ultimately keep to leaner dark' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'keep to leaner dark' are 'PORK'.
(Other definitions for pork that I've seen before include "Cassoulet ingredient" , "what we get from swine" , "Food -- hats -- falsehoods" , "Pig flesh" , "Meat used in chorizo" .)