Fellow anticipating full-scale quarrel (4,3)
I believe the answer is:
fall out
'quarrel' is the definition.
(falling out is a kind of quarrelling)
'fellow anticipating full-scale' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'anticipating' means one lot of letters go next to another.
'full-scale' becomes 'allout' ('all-out' can be a synonym of 'full-scale').
'f'+'allout'='FALL-OUT'
(Other definitions for fall out that I've seen before include "Happen" , "Radioactive pollution" , "disagree" , "Quarrel - leave the ranks" .)