A hatter is mad, essentially (2,5)
I believe the answer is:
at heart
'essentially' is the definition.
(I've seen this before)
'a hatter is mad' is the wordplay.
'is mad' indicates anagramming the letters.
'hatter' with letters rearranged gives 'theart'.
'a'+'theart'='AT HEART'
(Other definitions for at heart that I've seen before include "In one's real nature - a threat (anag)" , "Basically" , "real character" , "Essentially" , "A threat, in one's inmost feelings" .)