Appoint entirely unsatisfactory diplomat (15)
I believe the answer is:
plenipotentiary
'diplomat' is the definition.
(plenipotentiary is a kind of diplomat)
'appoint entirely unsatisfactory' is the wordplay.
'unsatisfactory' indicates an anagram (I've seen this in other clues).
'appoint'+'entirely'='appointentirely'
'appointentirely' with letters rearranged gives 'PLENIPOTENTIARY'.
(Other definitions for plenipotentiary that I've seen before include "Powerful diplomat" , ""The Ambassador"" .)