Sad as some music is cut short (4)
I believe the answer is:
blue
'sad' is the definition.
('blue' can be similar in meaning to 'sad')
'some music is cut short' is the wordplay.
'some music' becomes 'blues' (blues is a kind of music).
'is cut short' means to remove the last letter.
'blues' with its last letter taken away is 'BLUE'.
'as' is the link.
(Other definitions for blue that I've seen before include "Colour - smutty" , "Unsuitable for young" , "Sunny sky or low spirits" , "Between green and violet" , "Spend all at once - colour" .)