Bar Council pens instruction for one on the fiddle (4)
I believe the answer is:
arco
'instruction for one on the fiddle' is the definition.
(instruction to play a violin with a bow)
'bar council pens' is the wordplay.
'pens' indicates the answer is hidden within the clue (pen can mean to contain or cage).
'ARCO' can be found hidden inside 'bar council'.
(Other definitions for arco that I've seen before include "bow for players" , "which every violinist must grasp" , "Musical direction" , "Played with the bow" , "taking a bow" .)