Donation from whistle-blower backing politician (9)
I believe the answer is:
offertory
'donation' is the definition.
(offertory is a kind of donation)
'from whistle-blower backing politician' is the wordplay.
'from' becomes 'of' (I've seen this in other clues).
'whistle blower' becomes 'ref' (I've seen this in other clues).
'backing' shows that the letters should be reversed in order.
'politician' becomes 'tory' (I've seen this before).
'ref' reversed gives 'fer'.
'of'+'fer'+'tory'='OFFERTORY'
(Other definitions for offertory that I've seen before include "Fry to fore at this part of church ceremony" , "Something given" , "part of church service" , "Donation" , "Money collected during church service" .)