Impropriety of popular Italian novelist breaking cylinder (9)
I believe the answer is:
indecorum
'impropriety' is the definition.
('indecorum' can be a synonym of 'impropriety')
'popular italian novelist breaking cylinder' is the wordplay.
'popular' becomes 'in'.
'italian novelist' becomes 'eco' (I've seen this in other clues).
'breaking' means one lot of letters goes inside another.
'cylinder' becomes 'drum' (drum is a kind of cylinder).
'eco' going within 'drum' is 'decorum'.
'in'+'decorum'='INDECORUM'
'of' acts as a link.
(Other definitions for indecorum that I've seen before include "lack of decency!" , "unseemly behaviour" , "impropriety" , "Bad behaviour" , "bad taste" .)