Leading cook holds one (5)
I believe the answer is:
chief
'leading' is the definition.
(I've seen this in another clue)
'cook holds one' is the wordplay.
'cook' becomes 'chef' (both can mean someone who cooks).
'holds' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'chef' enclosing 'i' is 'CHIEF'.
(Other definitions for chief that I've seen before include "Tribal head" , "Tribe leader" , "Main" , "Boss" , "Person in charge, head" .)