Vague tirade with eminent lunatic ... (13)
I believe the answer is:
indeterminate
'vague' is the definition.
(I've seen this before)
'tirade with eminent lunatic' is the wordplay.
'with' is a charade indicator (letters next to each other).
'lunatic' indicates anagramming the letters.
'tirade' after 'eminent' is 'eminenttirade'.
'eminenttirade' anagrammed gives 'INDETERMINATE'.
(Other definitions for indeterminate that I've seen before include "Unspecified" , "not exactly fixed" , "Not precisely fixed" , "uncertain result?" , "Unfixed, uncertain" .)