Most important one entertained by cook (5)
I believe the answer is:
chief
'most important' is the definition.
(I've seen this in another clue)
'one entertained by cook' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entertained by' means one lot of letters goes inside another (inserted letters are hosted or entertained).
'cook' becomes 'chef' (both can mean someone who cooks).
'i' placed within 'chef' is 'CHIEF'.
(Other definitions for chief that I've seen before include "Head of a native American tribe" , "Tribal head" , "Leader; most important" , "Tribal leader" , "Gaffer" .)