It's sharp - no no, not the musical instrument! (5)
I believe the answer is:
thorn
'it's sharp' is the definition.
I can't judge whether this defines the answer.
'no no not the musical instrument' is the wordplay.
'no' is a deletion indicator.
'the musical instrument' becomes 'horn' (horn is a kind of musical instrument).
'not' with 'no' removed is 't'.
't'+'horn'='THORN'
Can you help me to learn more?
(Other definitions for thorn that I've seen before include "It causes irritation and annoyance" , "Prickle; annoyance" , "Pointed growth" , "Sharp spike on a plant" , "quill?" .)