Popular 1930s art? Strange lack of propriety! (9)
I believe the answer is:
indecorum
'lack of propriety' is the definition.
(I've seen this before)
'popular 1930s art? strange' is the wordplay.
I cannot really see how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found in the answer.
'strange' could be 'rum' (similar in meaning) and 'rum' is present in the answer.
The remaining letters 'deco' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for indecorum that I've seen before include "bad taste" , "Improper behaviour" , "impropriety" , "unseemly behaviour" , "Bad behaviour" .)