Demote ambassador backing heartless renegade (8)
I believe the answer is:
relegate
'demote' is the definition.
(I know that demote can be written as relegate)
'ambassador backing heartless renegade' is the wordplay.
'ambassador' becomes 'legate' (I've seen this in another clue).
'backing' says to put letters next to each other.
'heartless' says to hollow out the word (remove centre letters).
'renegade' with its centre taken out is 're'.
'legate' after 're' is 'RELEGATE'.
(Other definitions for relegate that I've seen before include "Demote" , "Move to a lower position" , "Assign to a lower position" , "Transfer to a lower division" , "Put in a lower division" .)