Bad month to start a year! (5)
I believe the answer is:
decay
'bad' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'month to start a year' is the wordplay.
'month' becomes 'dec' (abbreviation for December).
'to start' says to put letters next to each other.
'year' becomes 'y'.
'dec'+'a'+'y'='DECAY'
Can you help me to learn more?
(Other definitions for decay that I've seen before include "Go bad" , "Briefly fashionable" , "Putrefaction" , "Atrophy" , "Decompose or fall into ruin" .)