Menace of mad hatter? (6)
I believe the answer is:
threat
'menace' is the definition.
(synonyms)
'mad hatter?' is the wordplay.
'mad' is an anagram indicator.
'hatter' is an anagram of 'THREAT'.
'of' acts as a 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" .)