Avoid colloquial language on the barrack square (4)
I believe the answer is:
shun
'avoid colloquial language' is the definition.
'shun' can be an answer for 'avoid' (shunning is a kind of avoiding). I am not sure about the 'colloquial language' bit.
'on the barrack square' is the wordplay.
'on' says to put letters next to each other.
'the barrack' becomes 'hun' (I can't explain this - if you can you should give a lot more credence to this answer).
'square' becomes 's' (abbreviation).
'hun' put after 's' is 'SHUN'.
Can you help me to learn more?
(Other definitions for shun that I've seen before include "Avoid, ignore" , "Deliberately avoid" , "Keep away from, have nothint to do with" , "Avoid contact" , "Spurn; avoid" .)