Deal for one accepted by great advocate (9)
I believe the answer is:
supporter
'advocate' is the definition.
(supporter is a kind of advocate)
'deal for one accepted by great' is the wordplay.
'deal for' becomes 'port' (I can't explain this - if you can you should believe this answer much more).
'one accepted by' means one lot of letters goes inside another (inserted letters are accepted).
'great' becomes 'super' (both can mean excellent).
'port' put within '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" .)