Bad person above all others joins up (8)
I believe the answer is:
stinkpot
'bad person' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'above all others joins up' is the wordplay.
'above all others' becomes 'top' (I can't justify this - if you can you should give a lot more credence to this answer).
'joins' becomes 'knits' (knitting is a kind of joining).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'top'+'knits'='topknits'
'topknits' back-to-front is 'STINKPOT'.
Can you help me to learn more?
(Other definitions for stinkpot that I've seen before include "Unwholesome character" , "Unpleasant (-smelling?) person" , "turtle" .)