Deal for one accepted by great advocate (9)
I believe the answer is:
supporter
'advocate' is the definition.
(I know that supporter is a type of advocate)
'deal for one accepted by great' is the wordplay.
'deal for' becomes 'port' (I am not sure about this - if you are sure you should believe this answer much more).
'one accepted by' indicates putting letters inside (inserted letters are accepted).
'great' becomes 'super' (both can mean excellent).
'port' placed inside 'super' is 'SUPPORTER'.
Can you help me to learn more?
(Other definitions for supporter that I've seen before include "One helping" , "Advocate" , "Fan; heraldic figure" , "Adherent, defender" , "Strut" .)