Run down slow person for holding one up (8)
I believe the answer is:
denounce
'run down slow person for holding one up' is the definition.
I can't tell whether this defines the answer.
'for holding one up' is the wordplay.
'for' becomes 'dunce' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'holding' means one lot of letters goes inside another.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'one' written backwards gives 'eno'.
'dunce' enclosing 'eno' is 'DENOUNCE'.
Can you help me to learn more?
(Other definitions for denounce that I've seen before include "Hit out at" , "Speak publicly or give information against" , "As threat, identify" , "Accuse publicly" , "Impugn" .)