A kit for an aerosol-generating system, comprising: an aerosol-generating device and a compliant mouthpiece cover, the device comprising: a housing containing or configured to receive an aerosol-forming substrate; an outlet formed in the housing, wherein in use aerosol generated from the aerosol-forming substrate is delivered through the outlet; wherein the housing comprises a first wall surrounding the outlet and a second wall surrounding the first wall such that an open-ended annular recess is defined between the first and second walls, the annular recess being suitable for retaining the compliant mouthpiece cover to the housing; wherein the compliant mouthpiece cover comprises a tubular section configured to fit in the annular recess and being configured to allow aerosol delivered through the outlet to pass through the mouthpiece cover and into the mouth of a user in use; and wherein the aerosol-generating device is a smoking device, wherein the aerosol-generating device further comprises a tightening mechanism in the form of a resilient o-ring or helical spring received in a groove in the second wall, the o-ring or helical spring being configured to grip the mouthpiece cover received in the recess, and a slidable collar provided around the o-ring or spring which is moveable between a first position in which the collar does not compress the o-ring or helical spring and second position in which the collar compresses the o-ring or helical spring to grip the compliant mouthpiece cover when received in the annular recess, and wherein the kit comprises a plurality of mouthpiece covers. an aerosol-generating device and a compliant mouthpiece cover, the device comprising: a housing containing or configured to receive an aerosol-forming substrate; an outlet formed in the housing, wherein in use aerosol generated from the aerosol-forming substrate is delivered through the outlet; a housing containing or configured to receive an aerosol-forming substrate; an outlet formed in the housing, wherein in use aerosol generated from the aerosol-forming substrate is delivered through the outlet; wherein the housing comprises a first wall surrounding the outlet and a second wall surrounding the first wall such that an open-ended annular recess is defined between the first and second walls, the annular recess being suitable for retaining the compliant mouthpiece cover to the housing; wherein the compliant mouthpiece cover comprises a tubular section configured to fit in the annular recess and being configured to allow aerosol delivered through the outlet to pass through the mouthpiece cover and into the mouth of a user in use; and wherein the aerosol-generating device is a smoking device, wherein the aerosol-generating device further comprises a tightening mechanism in the form of a resilient o-ring or helical spring received in a groove in the second wall, the o-ring or helical spring being configured to grip the mouthpiece cover received in the recess, and a slidable collar provided around the o-ring or spring which is moveable between a first position in which the collar does not compress the o-ring or helical spring and second position in which the collar compresses the o-ring or helical spring to grip the compliant mouthpiece cover when received in the annular recess, and wherein the kit comprises a plurality of mouthpiece covers.