Bad behaviour once I'd mixed spirits (9)
I believe the answer is:
indecorum
'bad behaviour' is the definition.
(I've seen this before)
'once i'd mixed spirits' is the wordplay.
'mixed' indicates anagramming the letters.
'spirits' becomes 'rum' (rum is a kind of spirits).
'once'+'id'='onceid'
'onceid' anagrammed gives 'indeco'.
'indeco'+'rum'='INDECORUM'
(Other definitions for indecorum that I've seen before include "lack of decency!" , "unseemly behaviour" , "Naughty behaviour" , "bad taste" , "Bad behaviour" .)