English composer is heartless master of ceremonies (4)
I believe the answer is:
host
'master of ceremonies' is the definition.
('host' can be a synonym of 'master of ceremonies')
'english composer is heartless' is the wordplay.
'english composer' becomes 'holst' (I've seen this before).
'is heartless' says to hollow out the word (remove centre letters).
'holst' with its centre taken out is 'HOST'.
(Other definitions for host that I've seen before include "Multitude -- MC" , "Party giver" , "This man entertains" , "large group of people" , "Provide entertainment" .)