Club official more confident supporting base rate (9)
I believe the answer is:
treasurer
'club official' is the definition.
(I've seen this in another clue)
'more confident supporting base rate' is the wordplay.
'more confident' becomes 'surer' ('sure' can be similar in meaning to 'confident').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'base' indicates anagramming the letters (base can mean impure or corrupt).
'rate' with letters rearranged gives 'trea'.
'surer' put after 'trea' is 'TREASURER'.
(Other definitions for treasurer that I've seen before include "Committee member" , "Council official" , "Finance officer" , "Fund holder" , "Person holding purse" .)