Messenger bringing in tons for royal attendant (8)
I believe the answer is:
courtier
'royal attendant' is the definition.
(I've seen this before)
'messenger bringing in tons' is the wordplay.
'messenger' becomes 'courier' ('courier' can be a synonym of 'messenger').
'bringing in' is an insertion indicator.
'tons' becomes 't'.
'courier' enclosing 't' is 'COURTIER'.
'for' is the link.
(Other definitions for courtier that I've seen before include "Attendant on monarch" , "king's follower" , "who has Palace role?" , "Palace hanger-on" , "Sovereign's attendant" .)