Mad Hatter not hard to deal with (5)
I believe the answer is:
treat
'deal with' is the definition.
('treat' can be a synonym of 'deal with')
'mad hatter not hard' is the wordplay.
'mad' indicates anagramming the letters.
'not' is a deletion indicator.
'hard' becomes 'h'.
'hatter' with 'h' taken out is 'atter'.
'atter' anagrammed gives 'TREAT'.
'to' is the link.
(Other definitions for treat that I've seen before include "Pay for - minister to - deal with" , "An indulgence; handle" , "Enjoyable party" , "Pay for someone's food or drink or look after them medically" , "Source of gratification" .)