Free rebel at one by arrangement (8)
I believe the answer is:
liberate
'free' is the definition.
(I know that free can be written as liberate)
'rebel at one by arrangement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'by arrangement' indicates anagramming the letters.
'rebel'+'at'+'i'='rebelati'
'rebelati' with letters rearranged gives 'LIBERATE'.
(Other definitions for liberate that I've seen before include "Set free, rescue from bondage" , "Set free, release from slavery" , "Deliver" , "let go" , "Free (someone)" .)