A great deal about large soil additive (5)
I believe the answer is:
mulch
'soil additive' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'a great deal about large' is the wordplay.
'a great deal' becomes 'much' (synonyms).
'about' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'much' going around 'l' is 'MULCH'.
Can you help me to learn more?
(Other definitions for mulch that I've seen before include "Material for garden" , "It holds moisture and checks weeds" , "Decaying leaves, bark or compost spread around a plant" , "Garden dressing" , "Material laid on soil" .)