Gothic gong I've improperly seized (8)
I believe the answer is:
medieval
'gothic' is the definition.
(I know that gothic can be written as medieval)
'gong i've improperly seized' is the wordplay.
'gong' becomes 'medal' (gong can mean a prize or medal).
'improperly' is an anagram indicator (I've seen this in other clues).
'seized' means one lot of letters goes inside another.
'ive' anagrammed gives 'iev'.
'medal' going around 'iev' is 'MEDIEVAL'.
(Other definitions for medieval that I've seen before include "Of an earlier period" , "Belonging to the Middle Ages" , "Of the middle ages, or really old-fashioned" , "Am veiled in style of the middle ages" , "Relating to the Middle Ages" .)