Your writer confident around a music bar (7)
I believe the answer is:
measure
'music bar' is the definition.
(measure can mean a bar of music)
'your writer confident around a' is the wordplay.
'your writer' becomes 'me' (the person writing the clues for you).
'confident' becomes 'sure' (similar in meaning).
'around' means one lot of letters goes inside another.
'me'+'sure'='mesure'
'mesure' placed around 'a' is 'MEASURE'.
(Other definitions for measure that I've seen before include "Establish the size" , "Take size of" , "Tape" , "Determine the size of" , "Find the size" .)