First of all, I'm dismayed: I'm ostensibly the fool (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'first of all i'm dismayed i'm ostensibly the' is the wordplay.
'first of all' says to take the initial letters.
The first letters of 'im dismayed im ostensibly the' is 'IDIOT'.
(Other definitions for idiot that I've seen before include "Oaf" , "Dolt" , "Ass, ninny" , "Foolish person" , "Moron, fool" .)