Assurance from a percussionist looking over Mozart and Beethoven's overtures (6)
I believe the answer is:
aplomb
'assurance' is the definition.
(I've seen this before)
'from a percussionist looking over mozart and beethoven's overtures' is the wordplay.
'from' indicates an anagram (the anagram comes from the same letters).
'overtures' says to take the initial letters.
The first letters of 'percussionist looking over mozart and beethovens' is 'plomab'.
'plomab' anagrammed gives 'APLOMB'.
(Other definitions for aplomb that I've seen before include "Self-satisfied dignity, complacency" , "Self-confidence, assurance" , "Self-satisfied dignity and confidence" , "Air of self-satisfied confidence" , "Cool self-confidence" .)