Perfect in diplomacy (6)
I believe the answer is:
intact
'perfect' is the definition.
(I've seen this before)
'in diplomacy' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'in'+'tact'='INTACT'
(Other definitions for intact that I've seen before include "Complete - unbroken" , "Unbroken - sound" , "Flawless" , "Not damaged or missing a part" , "All in one piece" .)