Leader is top purchasing unfinished plant (9)
I believe the answer is:
executive
'leader' is the definition.
(I know that executive is a type of chief)
'top purchasing unfinished plant' is the wordplay.
'top' becomes 'execute' (I can't justify this - if you can you should give a lot more credence to this answer).
'purchasing' is an insertion indicator (some letters take possession of others).
'unfinished' means to remove the last letter.
'plant' becomes 'ivy' (ivy is a plant).
'ivy' with its final letter taken off is 'iv'.
'execute' enclosing 'iv' is 'EXECUTIVE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for executive that I've seen before include "Person who makes and carries out decisions" , "Branch of government charged with putting laws, agreements etc into effect" , "administration" , "Senior businessperson" , "Company director" .)