FA diplomacy backed magnate (3,3)
I believe the answer is:
fat cat
'magnate' is the definition.
(I've seen this before)
'fa diplomacy backed' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'backed' is a reversal indicator.
'tact' reversed gives 'tcat'.
'fa'+'tcat'='FAT CAT'
(Other definitions for fat cat that I've seen before include "Wealthy and privileged person" , "Wealthy and powerful person (informal)" , "Big cheese" , "Rich person or obese feline" , "Excessively wealthy person" .)