Fundamentally a mad hatter (2,5)
I believe the answer is:
at heart
'fundamentally' is the definition.
(I've seen this in another clue)
'a mad hatter' is the wordplay.
'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 "Basically" , "Substantially" , "In one's real nature - a threat (anag)" , "Essentially" , "In reality" .)