I must leave leading cook (4)
I believe the answer is:
chef
'cook' is the definition.
(both can mean someone who cooks)
'i must leave leading' is the wordplay.
'must leave' is a deletion indicator.
'leading' becomes 'chief' (I've seen this in another clue).
'chief' with 'i' taken away is 'CHEF'.
(Other definitions for chef that I've seen before include "Head cook in a restaurant" , "Hotel cook" , "Saucier" , "one in the restaurant" , "member of kitchen staff" .)