Go to pieces, some by Spanish composer (4,5)
I believe the answer is:
fall apart
'go to pieces' is the definition.
('fall apart' can be a synonym of 'go to pieces')
'some by spanish composer' is the wordplay.
'some' becomes 'part'.
'by' says to put letters next to each other.
'spanish composer' becomes 'falla' (Manuel de Falla).
'part' put after 'falla' is 'FALL APART'.
(Other definitions for fall apart that I've seen before include "Come to pieces" , "Go to pieces" , "Disintegrate" .)