Cook can't use fruitcake (7)
I believe the answer is:
nutcase
'fruitcake' is the definition.
(I've seen this before)
'cook can't use' is the wordplay.
'cook' indicates an anagram.
'cant'+'use'='cantuse'
'cantuse' anagrammed gives 'NUTCASE'.
(Other definitions for nutcase that I've seen before include "Eccentric" , "Slangily, a crazy person" , "bananas" , "fool" , "Crank" .)