Trumpet's first sound is false (11)
I believe the answer is:
treasonable
'trumpet's first' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'sound is false' is the wordplay.
I cannot quite see how this works, but
'sound' could be 'reasonable' (similar in meaning) and 'reasonable' is located in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for treasonable that I've seen before include "criminal" , "Worse than seditious" , "Guilty of betrayal" , "(Of offence) relating to betrayal of one's country" , "giving people away" .)