She's cracked a nut (4)
I believe the answer is:
aunt
'she's' is the definition.
('she' could refer to an aunt)
'cracked a nut' is the wordplay.
'cracked' indicates anagramming the letters.
'a'+'nut'='anut'
'anut' with letters rearranged gives 'AUNT'.
(Other definitions for aunt that I've seen before include "Tuna gone bad for the relative" , "Parent's female sibling" , "Tuna (anag.)" , "Family relative" , "Put up with" .)