Composer Bishop Marsh slammed (6)
I believe the answer is:
brahms
'composer' is the definition.
(I know that Brahms is an composer)
'bishop marsh slammed' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'slammed' indicates an anagram.
'marsh' is an anagram of 'rahms'.
'b'+'rahms'='BRAHMS'
(Other definitions for brahms that I've seen before include "drunk?" , "Composer of a well-known lullaby published in 1868" , "19th century German composer, known for Lullaby" , "Johannes . . . . . ., German composer of famous lullaby" , "Composer (associated with Liszt?)" .)