Fair, somewhat slim and not all there! (9)
I believe the answer is:
impartial
'fair' is the definition.
(associated in meaning)
'somewhat slim and not all there' is the wordplay.
'somewhat slim' becomes 'im' (I am not sure about this - if you are sure you should believe this answer much more).
'and' is a charade indicator (letters next to each other).
'not all there' becomes 'partial' (I've seen this in another clue).
'im'+'partial'='IMPARTIAL'
Can you help me to learn more?
(Other definitions for impartial that I've seen before include "Unbiased" , "Treating all rivals equally" , "Showing lack of favouritism" , "Fair, unprejudiced" , "Disinterested" .)