Messenger in hall of justice endlessly? That is right (7)
I believe the answer is:
courier
'messenger' is the definition.
('courier' can be a synonym of 'messenger')
'hall of justice endlessly? that is right' is the wordplay.
'hall of justice' becomes 'court' (I've seen this before).
'endlessly?' means to remove the last letter (I've seen 'endlessly' mean this).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'right' becomes 'r' (common abbreviation).
'court' with its final letter taken away is 'cour'.
'cour'+'ie'+'r'='COURIER'
'in' acts as a link.
(Other definitions for courier that I've seen before include "Type" , "Messenger conveying news or document (7)" , "Travel guide" , "Tourist guide" , "Message-carrier" .)