Mad Hatter creates a risk (6)
I believe the answer is:
threat
'a risk' is the definition.
(I've seen this before)
'mad hatter' is the wordplay.
'mad' is an anagram indicator.
'hatter' with letters rearranged gives 'THREAT'.
'creates' acts as a link.
(Other definitions for threat that I've seen before include "Aggressive declaration" , "An intimidatory warning" , "Warning of intent to harm" , "Hatter (anag)" , "Intimidatory words to exact compliance" .)