Mad Hatter is taking on top end counsellor (9)
I believe the answer is:
therapist
'counsellor' is the definition.
'mad hatter is taking on top end' is the wordplay.
'mad' indicates an anagram.
'taking on' means one lot of letters goes inside another.
'end' indicates one should take the final letters.
The last letter of 'top' is 'p'.
'hatter'+'is'='hatteris'
'hatteris' is an anagram of 'theraist'.
'theraist' going around 'p' is 'THERAPIST'.
(Other definitions for therapist that I've seen before include "See 21" , "One treating disease" , "nurse" , "Healer of disease" , "provider of treatment" .)