Flipping saint has a hundred grand! (9)
I believe the answer is:
switching
'flipping' is the definition.
('switch' can be a synonym of 'flip')
'saint has a hundred grand' is the wordplay.
'saint' becomes 'swithin' (saint swithun/swithin).
'has' indicates putting letters inside.
'a hundred' becomes 'C' (Roman numerals).
'grand' becomes 'g' (G is short for grand as in a thousand).
'swithin' going around 'c' is 'switchin'.
'switchin'+'g'='SWITCHING'
(Other definitions for switching that I've seen before include "Transferring to another track - or whipping" , "Changing over" .)