Not the twit who's thin on top? (7)
I believe the answer is:
fathead
'not the twit' is the definition.
I can't tell whether this defines the answer.
'who's thin on top?' is the wordplay.
'who's thin' becomes 'fat' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'top?' becomes 'head' ('head' can be a synonym of 'top').
'fat'+'head'='FATHEAD'
Can you help me to learn more?
(Other definitions for fathead that I've seen before include "Charlie" , "Slang idiot with enlarged cranium?" , "silly ass!" , "Dimwit" , "Fool" .)