I get so nasty as self-centred person (6)
I believe the answer is:
egoist
'self-centred person' is the definition.
(I've seen this before)
'i get so nasty' is the wordplay.
'nasty' indicates anagramming the letters.
'i'+'get'+'so'='igetso'
'igetso' with letters rearranged gives 'EGOIST'.
'as' acts as a link.
(Other definitions for egoist that I've seen before include "He puts himself first" , "Bighead" , "self-seeking type" , "It goes with self-centred man" , "Narcissist" .)