Language Sean misused in jest (8)
I believe the answer is:
'language' is the definition.
(Japanese is an example)
'sean misused in jest' is the wordplay.
'misused' indicates an anagram.
'in' indicates putting letters inside.
'jest' becomes 'jape' (synonyms).
'sean' anagrammed gives 'anes'.
'anes' placed into 'jape' is 'JAPANESE'.
(Other definitions for japanese that I've seen before include "Person from Tokyo, say" , "Far easterner(s)" , "Eg, native to Tokyo" , "From Honshu, perhaps" , "This nationality drives NASA jeep badly" .)