Blunder dropping new musical instrument (5)
I believe the answer is:
bugle
'musical instrument' is the definition.
(bugle is a kind of musical instrument)
'blunder dropping new' is the wordplay.
'blunder' becomes 'bungle' (both can mean a mistake).
'dropping' indicates named letters should be taken away.
'new' becomes 'n' (abbreviation as in New Testament NT).
'bungle' with 'n' taken out is 'BUGLE'.
(Other definitions for bugle that I've seen before include "A military trumpet" , "Brass instrument without valves" , "costume ornament?" , "'Last Post' instrument" , "one may have blown it!" .)