Choir a man trained in mouth organ (9)
I believe the answer is:
harmonica
'mouth organ' is the definition.
(synonyms)
'choir a man trained' is the wordplay.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'choir'+'a'+'man'='choiraman'
'choiraman' anagrammed gives 'HARMONICA'.
'in' is the link.
(Other definitions for harmonica that I've seen before include "Larry Adler's instrument" , "Harp" , "Mouth organ" , "musical instrument" , "An alternative mouth-organ" .)