Diplomatic record king rejects regularly (8)
I believe the answer is:
discreet
'diplomatic' is the definition.
(I've seen this before)
'record king rejects regularly' is the wordplay.
'record' becomes 'disc' (eg a CD, compact disc).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'rejects' are 'eet'.
'disc'+'r'+'eet'='DISCREET'
(Other definitions for discreet that I've seen before include "Doesn't give away secrets - can be trusted" , "Unobtrusive; careful and prudent" , "Modest" , "Unobtrusive, circumspect" , "Careful to avoid embarrassment" .)