The nub is that uncle is out to get us (7)
I believe the answer is:
'the nub is that' is the definition.
'nucleus' can be an answer for 'the nub' (thesaurus). I'm not certain of the rest of the definition.
'uncle is out to get us' is the wordplay.
'is out' indicates an anagram (out can mean wrong or inaccurate).
'to get' says to put letters next to each other.
'uncle' is an anagram of 'nucle'.
Can you help me to learn more?
(Other definitions for nucleus that I've seen before include "Tiny central part of an atom" , "Heart of the Matter" , "Core, centre" , "Central mass of a cell" , "Atom's core" .)