Relative cracked a nut (4)
I believe the answer is:
aunt
'relative' is the definition.
(aunt is a kind of relative)
'cracked a nut' is the wordplay.
'cracked' indicates anagramming the letters.
'a'+'nut'='anut'
'anut' is an anagram of 'AUNT'.
(Other definitions for aunt that I've seen before include "Tuna gone bad for the relative" , "Female relative" , "Parent's female sibling" , "Tuna (anag.)" , "Female relation" .)