Argument against joining encounter (3,4)
I believe the answer is:
run into
'encounter' is the definition.
(I know that encounter can be written as run into)
'argument against joining' is the wordplay.
'argument' becomes 'run-in' (I've seen this before).
'against' becomes 'to' (as in a lean-to, building leaning against anothe).
'joining' is a charade indicator (letters next to each other).
'runin'+'to'='RUN INTO'
(Other definitions for run into that I've seen before include "Happen to meet or collide with" , "Come across, encounter" , "Happen to meet - with a bump?" , "charge?" , "Meet (a person) by chance" .)