One who doesn't believe that is wrong about her heart (7)
I believe the answer is:
atheist
'one who doesn't believe' is the definition.
'that is wrong about her heart' is the wordplay.
'wrong' indicates anagramming the letters.
'about' indicates putting letters inside.
'heart' says to take the centre.
The central letter of 'her' is 'e'.
'that'+'is'='thatis'
'thatis' with letters rearranged gives 'athist'.
'athist' placed around 'e' is 'ATHEIST'.
(Other definitions for atheist that I've seen before include "Person who believes there is no God" , "Unbeliever" , "One who disbelieves the existence of gods" , "Disbeliever in deities" , "Unfaithful type" .)