Quarrel over some nuclear issue (4,3)
I believe the answer is:
fall out
This might be a double definition.
'quarrel' is the first definition.
(I know that fall out is a more specific form of the action quarrel)
'some nuclear issue' is the second definition.
'fall out' can be an answer for 'issue' (thesaurus). I am not sure about the 'some nuclear' bit.
'over' is the link.
Can you help me to learn more?
(Other definitions for fall out that I've seen before include "Quarrel - leave the ranks" , "disagree" , "Radioactive pollution" , "Happen" .)