Very bad day, one covered by last month (7)
I believe the answer is:
demonic
'very bad' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?
'day one covered by last month' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'one' becomes 'i' (Roman numeral).
'covered by' indicates putting letters inside.
'last month' becomes 'dec' (short for December, last month of the year).
'mon'+'i'='moni'
'moni' put inside 'dec' is 'DEMONIC'.
Can you help me to learn more?
(Other definitions for demonic that I've seen before include "As if possessed by devil" , "Like evil spirits" , "Cruel like the devil" , "Furious" , "diabolical" .)