Preparing cocoa but heartlessly leaves (7)
I believe the answer is:
tobacco
'leaves' is the definition.
(I've seen this before)
'preparing cocoa but heartlessly' is the wordplay.
'preparing' indicates anagramming the letters.
'heartlessly' says to hollow out the word (remove centre letters).
'but' with its centre taken out is 'bt'.
'cocoa'+'bt'='cocoabt'
'cocoabt' with letters rearranged gives 'TOBACCO'.
(Other definitions for tobacco that I've seen before include "make a cot cob from this plant" , "Popular narcotic" , "Maybe Virginia" , "Snuff, for example" , "Smoke the cob taco" .)