Three times the highest pitch (6)
I believe the answer is:
treble
'pitch' is the definition.
(treble is a kind of pitch)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for treble that I've seen before include "Member of choir" , "Feat of winning three major trophies" , "High voice - large drink" , "High voice; tripartite" , "High boy's singing voice" .)