Again send most of the remainder with it (6)
I believe the answer is:
reship
'again send' is the definition.
('ship' can be a synonym for 'send' so perhaps 'reship' is OK for 'again send')
'most of the remainder with it' is the wordplay.
'most of' means to remove the last letter.
'the remainder' becomes 'rest' ('rest' can be a synonym of 'remainder').
'with it' becomes 'hip' (I've seen this in other clues).
'rest' with its final letter taken off is 'res'.
'res'+'hip'='RESHIP'
(Other definitions for reship that I've seen before include "Dispatch by sea again" , "send out again" , "move for a second time" .)