Items Spanish composer has to leave — the centre cannot hold: why not? (6,4,5)
I believe the answer is:
things fall apart
'the centre cannot hold why not?' is the definition.
('Things fall apart, the centre cannot hold' -Yeats)
'items spanish composer has to leave' is the wordplay.
'items' becomes 'things' (I've seen this before).
'spanish composer' becomes 'fal la' (Manuel de Falla).
'has' is a charade indicator (letters next to each other).
'to leave' becomes 'part' (part can mean to leave or depart).
'things'+'falla'+'part'='THINGS FALL APART'
(Other definitions for things fall apart that I've seen before include "result of poor manufacture" , "Mere anarchy follows this" .)