Clue: Leading trumpeter and percussionist missed out — appreciate playing as they wished (1,7)
Answer: A PIACERE
Explanation: Definition: as they wished
An anagram ('playing') of 'a[p]precia[t]e' minus T and P ('leading **T**rumpeter and **P**ercussionist missed out'), for a musical instruction similar to ad lib, indicating that the performer may choose the tempo, for example, at pleasure.