Unseemly behaviour in kind of art spirit (9)
I believe the answer is:
indecorum
'unseemly behaviour' is the definition.
(I've seen this before)
'in kind of art spirit' is the wordplay.
'kind of art' becomes 'deco'.
'spirit' becomes 'rum' (alcoholic spirit).
'in'+'deco'+'rum'='INDECORUM'
(Other definitions for indecorum that I've seen before include "Bad behaviour" , "Naughty behaviour" , "impropriety" , "Improper behaviour" , "bad taste" .)