Brother outed sham composer (6)
I believe the answer is:
brahms
'composer' is the definition.
(Brahms is an example)
'brother outed sham' is the wordplay.
'brother' becomes 'br' (abbreviation in titles of monks).
'outed' indicates an anagram.
'sham' anagrammed gives 'ahms'.
'br'+'ahms'='BRAHMS'
(Other definitions for brahms that I've seen before include "'Bah, Mrs, he's a composer (6)'" , "Composer of A German Requiem" , "Johannes . . . . . ., German composer of famous lullaby" , "German scorer" , "German barman" .)