Diplomat tersely concealing affairs (7)
I believe the answer is:
matters
'affairs' is the definition.
('matter' can be a synonym of 'affair')
'diplomat tersely concealing' is the wordplay.
'concealing' says the answer is hidden in the clue.
'MATTERS' is hidden within 'diplomat tersely'.
(Other definitions for matters that I've seen before include "Is important" , "Has import, counts" , "Is of importance" , "Topics under consideration" , "affairs" .)