Ultimately we cull musk deer (3)
I believe the answer is:
elk
'deer' is the definition.
(type of deer)
'ultimately we cull musk' is the wordplay.
'ultimately' indicates one should take the final letters.
The final letters of 'we cull musk' are 'ELK'.
(Other definitions for elk that I've seen before include "Fraternal American" , "Large ruminant" , "Large northern deer" , "Largest living deer" , "Asian moose" .)