Publicly accuse a stupid person holding one back (8)
I believe the answer is:
denounce
'publicly accuse' is the definition.
(I've seen this before)
'a stupid person holding one back' is the wordplay.
'a stupid person' becomes 'dunce' (I've seen this before).
'holding' means one lot of letters goes inside another.
'back' says the letters should be written in reverse.
'one' in reverse letter order is 'eno'.
'dunce' going around 'eno' is 'DENOUNCE'.
(Other definitions for denounce that I've seen before include "Speak publicly against, expose" , "Condemn publicly" , "Publicly declare to be wrong" , "Revile" , "attack" .)