Menace of mad hatter? (6)
I believe the answer is:
threat
'menace' is the definition.
(synonyms)
'mad hatter?' is the wordplay.
'mad' indicates an anagram.
'hatter' is an anagram of 'THREAT'.
'of' is the link.
(Other definitions for threat that I've seen before include "The rat became a menace" , "Endangerment" , "Statement of harmful intent" , "Intimidation or warning" , "Aggressive declaration" .)