Doing as ordered one's coming to conclusion (9)
I believe the answer is:
'conclusion' is the definition.
(a diagnosis is a conclusion or verdict)
'doing as ordered one's coming to' is the wordplay.
'ordered' indicates anagramming the letters.
'one's' becomes 'I's' (Roman numeral).
'coming to' is a charade indicator (letters next to each other).
'doingas' with letters rearranged gives 'diagnos'.
(Other definitions for diagnosis that I've seen before include "Identification of disease" , "analysis" , "Identification of a problem from its symptoms" , "Identification of an illness" , "Interpretation of" .)