Heart of uncle, upset with us (7)
I believe the answer is:
nucleus
'heart' is the definition.
(I've seen this before)
'uncle upset with us' is the wordplay.
'upset with' indicates anagramming the letters.
'uncle' anagrammed with 'us' can make 'NUCLEUS'.
'of' is the link.
(Other definitions for nucleus that I've seen before include "Centre of an atom" , "Heart of the Matter" , "basis for development" , "part of comet's head" , "Tiny central part of an atom" .)