The Mad Hatter gets a warning (6)
I believe the answer is:
threat
'a warning' is the definition.
(I know that threat is a type of warning)
'the mad hatter' is the wordplay.
'the mad' is an anagram indicator.
'hatter' with letters rearranged gives 'THREAT'.
'gets' acts as a link.
(Other definitions for threat that I've seen before include "An intimidating warning" , "Intimidation or warning" , "Expression of intention to harm" , "Menacing words" , "Hatter (anag)" .)