Patron has been upset over agent (10)
I believe the answer is:
benefactor
'patron' is the definition.
(I know that patron can be written as benefactor)
'been upset over agent' is the wordplay.
'upset' indicates an anagram.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'agent' becomes 'factor' (business term for a representative).
'been' with letters rearranged gives 'bene'.
'bene'+'factor'='BENEFACTOR'
'has' is the link.
(Other definitions for benefactor that I've seen before include "One giving money to a cause" , "charity supporter?" , "Patronising sort" , "One giving to worthy cause" , "Patron, sponsor" .)