Rule out piece of music containing bathetic ending (8)
I believe the answer is:
preclude
'rule out' is the definition.
('preclude' can be a synonym of 'rule out')
'piece of music containing bathetic ending' is the wordplay.
'piece of music' becomes 'prelude' (brief piece of music).
'containing' means one lot of letters goes inside another.
'ending' indicates one should take the final letters.
The final letter of 'bathetic' is 'c'.
'prelude' placed around 'c' is 'PRECLUDE'.
(Other definitions for preclude that I've seen before include "Debar" , "Obviate, prevent" , "Make impossible" , "'Prevent, forestall (8)'" , "bar" .)