Speed indicated at the start of tedious étude musician played, ostentatiously! (5)
I believe the answer is:
tempo
'speed' is the definition.
(I've seen this in another clue)
'start of tedious etude musician played ostentatiously' is the wordplay.
'start of' indicates taking the first letters.
The first letters of 'tedious etude musician played ostentatiously' is 'TEMPO'.
'indicated at the' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tempo that I've seen before include "Rhythm, metre" , "Pace - pacing" , "Speed at which a musical composition is to be played" , "Cadence" , "Rate and rhythm in music" .)