Frenchman Lehár trained as composer (6)
I believe the answer is:
mahler
'composer' is the definition.
(Gustav Mahler)
'frenchman lehar trained' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'trained' indicates an anagram (to train can mean to grow something into a shape).
'lehar' is an anagram of 'ahler'.
'm'+'ahler'='MAHLER'
'as' acts as a link.
(Other definitions for mahler that I've seen before include "Gustav . . . . . ., Austrian composer" , "Harlem (anag) - composer" , "Austrian composer from Harlem?" , "Interrupt; hang" , "Lift" .)