Defeat the endless argument supporting deliveries (9)
I believe the answer is:
overthrow
'defeat' is the definition.
(I've seen this before)
'the endless argument supporting deliveries' is the wordplay.
'endless' means to remove the last letter.
'argument' becomes 'row' (I've seen this before).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'deliveries' becomes 'over' (I've seen this before).
'the' with its final letter removed is 'th'.
'th'+'row'='throw'
'throw' after 'over' is 'OVERTHROW'.
(Other definitions for overthrow that I've seen before include "Defeat completely" , "Extra run perhaps" , "Worst" , "Dethrone or oust" , "Subvert" .)