Heartless leader becomes cook (4)
I believe the answer is:
chef
'cook' is the definition.
(chef is a kind of cook)
'heartless leader' is the wordplay.
'heartless' means to remove the middle letters.
'leader' becomes 'chief' (chief is a kind of leader).
'chief' with its middle removed is 'CHEF'.
'becomes' is the link.
(Other definitions for chef that I've seen before include "Head cook in a restaurant" , "Saucier" , "French master" , "Hotel cook" , "One who prepares courses" .)