Again appoint donkey in a rule (8)
I believe the answer is:
reassign
'again appoint' is the definition.
('assign' is an answer for 'appoint' so perhaps 'reassign' is OK for 'again appoint')
'donkey in a rule' is the wordplay.
'donkey' becomes 'ass' (ass is a term for a donkey).
'in' means one lot of letters goes inside another.
'a rule' becomes 'reign' (reign is a kind of rule).
'ass' inserted into 'reign' is 'REASSIGN'.
(Other definitions for reassign that I've seen before include "put in new position" , "put in another place" , "Give out afresh" , "change duties" , "Allocate again" .)