We're wrong about a bonehead 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 a bonehead' is the wordplay.
'wrong' indicates an anagram.
'about' means one lot of letters goes inside another.
'a bonehead' becomes 'b' (head letter of 'bone').
'were' is an anagram of 'weer'.
'weer' going around 'b' is 'WEBER'.
'being' is the link.
(Other definitions for weber that I've seen before include "Composer; physicist" , "bit of magnetism" , "physics unit" , "Composer of Der Freischutz, d. 1826" , "Founder of the German Romantic school of opera, d. 1826" .)