Proper behaviour regarding environment when absorbed by percussion instrument (7)
I believe the answer is:
decorum
'proper behaviour' is the definition.
'decorum' can be an answer for 'behaviour' (decorum is a kind of behaviour). I am unsure of the 'proper' bit.
'regarding environment when absorbed by percussion instrument' is the wordplay.
'regarding environment when' becomes 'eco' (I can't explain this - if you can you should believe this answer much more).
'absorbed by' means one lot of letters goes inside another.
'percussion instrument' becomes 'drum' (drum is a kind of percussion instrument).
'eco' inserted into 'drum' is 'DECORUM'.
Can you help me to learn more?
(Other definitions for decorum that I've seen before include "Protocol" , "Decency" , "Good manners" , "Proper behaviour; etiquette" , "modesty" .)