Quarrel with toxic consequences (4,3)
I believe the answer is:
fall out
'quarrel with toxic consequences' is the definition.
'fall out' can be an answer for 'quarrel' (falling out is a kind of quarrelling). I am unsure of the 'with toxic consequences' bit.
'quarrel with toxic consequences' is the wordplay.
'quarrel' becomes 'fall' (I've seen this before).
'with toxic consequences' becomes 'out' (I can't justify this - if you can you should believe this answer much more).
'fall'+'out'='FALL-OUT'
Can you help me to learn more?
(Other definitions for fall out that I've seen before include "disagree" , "Happen" , "Quarrel - leave the ranks" , "Radioactive pollution" .)