Dead time after amusement in December (7)
I believe the answer is:
defunct
'dead' is the definition.
(similar in meaning)
'time after amusement in december' is the wordplay.
'time' becomes 't' (abbreviation).
'after' means one lot of letters go next to another.
'amusement' becomes 'fun' (I've seen this before).
'in' means one lot of letters goes inside another.
'december' becomes 'dec' (synonyms).
'fun' going into 'dec' is 'defunc'.
't' after 'defunc' is 'DEFUNCT'.
(Other definitions for defunct that I've seen before include "Out of fashion, dead" , "No longer in use" , "No longer alive" , "Inoperative" , "No longer living or in force" .)