Modern diplomacy's not impaired (6)
I believe the answer is:
intact
'not impaired' is the definition.
'modern diplomacy's' is the wordplay.
'modern' becomes 'in' (I've seen this in other clues).
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'in'+'tact'='INTACT'
(Other definitions for intact that I've seen before include "Complete - unbroken" , "Entire, untouched" , "Not damaged or missing a part" , "without flaw?" , "Undiminished" .)