Organic material from our group supporting the sound of activity (5)
I believe the answer is:
humus
'organic material' is the definition.
(component of soil)
'our group supporting the sound of activity' is the wordplay.
'our group' becomes 'us'.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'the sound of activity' becomes 'hum' (eg the sound of a machine).
'us' after 'hum' is 'HUMUS'.
'from' acts as a link.
(Other definitions for humus that I've seen before include "Organic component of soil" , "Decomposed vegetation" , "Organic matter in topsoil" , "garden material" , "Rich organic material from dead leaves, plants etc" .)