Insult lawyers and bishop (4)
I believe the answer is:
Here is my best explanation:
'insult' is the definition.
(I've seen this before)
'lawyers and bishop' is the subsidiary indication.
'lawyers' becomes 'bar'.
'and' says to put letters next to each other.
'bishop' becomes 'b' (abbreviation used in chess).
(Other definitions for barb that I've seen before include "Insult", "Backward-pointing part of a fish hook", "Cutting remark", "Kind of thorn or wounding remark", "Sharp point; wounding comment", "Jibe", "Jag on a hook", "Stage whispers", "Wounding remark" and "Small prong on larger point".)