Leader quitting by arrangement (8)
I believe the answer is:
ordering
'arrangement' is the definition.
(ordering is a kind of arrangement)
'leader quitting by' is the wordplay.
'leader quitting' means to remove the first letter.
'by' becomes 'bordering'.
'bordering' with its initial letter removed is 'ORDERING'.
(Other definitions for ordering that I've seen before include "procurement" , "Requesting (something) to be supplied" , "Instructing" , "Issuing instructions" , "Arranging or asking for delivery of goods" .)