Mad Hatter, heartless even, shows menace (8)
I believe the answer is:
'menace' is the definition.
('threaten' can be a synonym of 'menace')
'mad hatter heartless even' is the wordplay.
'mad' is an anagram indicator.
'heartless' says to hollow out the word (remove centre letters).
'even' with its middle taken out is 'en'.
'hatter' is an anagram of 'threat'.
'shows' is the link.
(Other definitions for threaten that I've seen before include "Be menacing" , "Warn of violence" , "indicate danger" , "Cow" , "Put in jeopardy" .)