Uncle's wife is a nut, kind of (4)
I believe the answer is:
aunt
'uncle's wife' is the definition.
(I've seen this before)
'a nut kind of' is the wordplay.
'kind of' indicates an anagram.
'nut' is an anagram of 'unt'.
'a'+'unt'='AUNT'
'is' acts as a link.
(Other definitions for aunt that I've seen before include "Put up with" , "Family relative" , "Tuna (anag.)" , "Charley's in play?" , "Parent's female sibling" .)