Indecent, unseemly (8)
I believe the answer is:
I believe this clue is a double definition.
'indecent' is the first definition.
(associated in meaning)
'unseemly' is the second definition.
(similar in meaning)
(Other definitions for improper that I've seen before include "unfit" , "Wrong - unsuitable" , "'Not suitable, right or appropriate (8)'" , "Incorrect or wrong, say behaviour" , "Unseemly, indelicate" .)