Mad Hatter's menaces (7)
I believe the answer is:
threats
'menaces' is the definition.
('threat' can be a synonym of 'menace')
'mad hatter's' is the wordplay.
'mad' indicates an anagram.
'hatters' with letters rearranged gives 'THREATS'.
(Other definitions for threats that I've seen before include "Shatter (anag) - menaces" , "Statements of intentions to harm" , "Menacing statements" , "Sources of danger -- shatter (anag)" , "Expressions of intention to harm" .)