To be fair, I'm not all there! (9)
I believe the answer is:
impartial
'to be fair' is the definition.
(synonyms)
'i'm not all there' is the wordplay.
'not all there' becomes 'partial' (I've seen this in another clue).
'im'+'partial'='IMPARTIAL'
(Other definitions for impartial that I've seen before include "not taking sides" , "Unbiased" , "Disinterested" , "Showing lack of favouritism" , "Fair, unprejudiced" .)