Cook some brunch effectively (4)
I believe the answer is:
chef
'cook' is the definition.
(chef is a kind of cook)
'some brunch effectively' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'CHEF' can be found hidden inside 'brunch effectively'.
(Other definitions for chef that I've seen before include "Key kitchen worker" , "Hotel cook" , "French master" , "Kitchen head" , "Chief male cook in restaurant" .)