Notes a day out to be a bad thing! (5)
I believe the answer is:
decay
'a bad thing' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'notes a day out' is the wordplay.
'notes' becomes 'ec' ().
'out' is an insertion indicator.
'ec' inserted inside 'day' is 'DECAY'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for decay that I've seen before include "Deterioration" , "Go bad" , "'Decompose, rot (5)'" , "Putrefaction" , "Atrophy" .)