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