Crazy cooking keeps duck cold (6)
I believe the answer is:
coryza
'cold' is the definition.
(I've seen this before)
'crazy cooking keeps duck' is the wordplay.
'cooking' indicates an anagram (cook can mean to falsify or distort).
'keeps' means one lot of letters goes inside another.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'crazy' with letters rearranged gives 'cryza'.
'cryza' placed around 'o' is 'CORYZA'.
(Other definitions for coryza that I've seen before include "Term for cold in the head" , "Nasal inflammation" , "Catarrh" , "effect of cold" .)