Self-important pundit's first sin is to support United (6)
I believe the answer is:
'self-important' is the definition.
(I've seen this before)
'pundit's first sin is to support united' is the wordplay.
'pundit's first' becomes 'p' (pst letter of 'pundit').
'sin' becomes 'pity' (I can't explain this - if you can you should believe this answer much more).
'is to support' says to put letters next to each other (in a down clue, some letters hold up others).
'united' becomes 'U' (abbreviation).
'ppity' after 'u' is 'UPPITY'.
Can you help me to learn more?
(Other definitions for uppity that I've seen before include "Informally presumptuous or arrogant" , "In need of slapping down?" , "Difficult to control" , "Pert; haughty" , "Informally pert or arrogant" .)