Reject article essentially congratulating disagreeable personality (6)
I believe the answer is:
nature
'personality' is the definition.
(thesaurus)
'reject article essentially congratulating disagreeable' is the wordplay.
'reject' says the letters should be written backwards.
'article' becomes 'an' (indefinite article).
'essentially' says to take the centre.
'disagreeable' becomes 're' (I can't explain this - if you can you should believe this answer much more).
The central letters of 'congratulating' are 'tu'.
'an' backwards is 'na'.
'na'+'tu'+'re'='NATURE'
(Other definitions for nature that I've seen before include "mother, so-called" , "Innate quality" , "All non-artificial phenomena" , "Fundamental character" , "Kind, sort; countryside" .)