A year after “essential,” stale (5)
I believe the answer is:
musty
'stale' is the definition.
(similar in meaning)
'a year after essential' is the wordplay.
'a year' becomes 'y'.
'after' says to put letters next to each other.
'essential' becomes 'must' (similar in meaning).
'y' after 'must' is 'MUSTY'.
(Other definitions for musty that I've seen before include "Frowsty" , "Mouldy; stale-smelling" , "Stale and dank" , "Smelling stale or damp" , "Mouldy; outdated" .)