We're wrong about Bach initially being a composer! (5)
I believe the answer is:
weber
'a composer' is the definition.
(Carl Maria von Weber was a German composer)
'we're wrong about bach initially' is the wordplay.
'wrong' is an anagram indicator.
'about' means one lot of letters goes inside another.
'initially' suggests taking the first letters.
The first letter of 'bach' is 'b'.
'were' with letters rearranged gives 'weer'.
'weer' enclosing 'b' is 'WEBER'.
'being' acts as a link.
(Other definitions for weber that I've seen before include "Founder of the German Romantic school of opera, d. 1826" , "bit of magnetism" , "Unit of magnetic flux, composer" , "Opera composer" , "Composer of Der Freischutz, d. 1826" .)