Crossword Genius

Fundamentally a mad hatter (2,5)

Ross

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" .)

I've seen this clue in The Times.
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