Kind of family crazy uncle and aunt rule, primarily (7)
I believe the answer is:
nuclear
'kind of family' is the definition.
(nuclear family is a term for a kind of family)
'crazy uncle and aunt rule primarily' is the wordplay.
'crazy' indicates an anagram.
'and' says to put letters next to each other.
'primarily' says to take the initial letters.
The first letters of 'aunt rule' is 'ar'.
'uncle' with letters rearranged gives 'nucle'.
'nucle'+'ar'='NUCLEAR'
(Other definitions for nuclear that I've seen before include "basic family" , "Relating to atomic power" , "Relating to atomic energy" , "Using atomic energy" , "Relating to a core" .)