Are most trained by an expert? (7)
I believe the answer is:
maestro
'an expert?' is the definition.
(I've seen this before)
'are most trained' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'are'+'most'='aremost'
'aremost' is an anagram of 'MAESTRO'.
'by' is the link.
(Other definitions for maestro that I've seen before include "Distinguished composer, conductor or performer" , "An artist of consummate skill, master" , "musical genius" , "Genius possibly" , "hotshot" .)