Queue at the pub? Musician will be familiar with it (3,4)
I believe the answer is:
bar line
'musician will be familiar with it' is the definition.
(line on a musical score)
'queue at the pub?' is the wordplay.
'queue' becomes 'line' (a line of people).
'at' says to put letters next to each other (I've seen this in other clues).
'the pub?' becomes 'bar' (both are drinking establishments).
'line' put after 'bar' is 'BAR LINE'.
(Other definitions for bar line that I've seen before include "Vertical in a music score" , "Mark in musical score" , "Division in a musical score" .)