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 "Hurtful witticism - on wire", "added to wire?", "criticism", "Thorn or spike", "Fish", "Sharp projection", "Arrow projection", "Slight", "Sharp point; wounding comment" and "A thorn or a gibe".)